diff --git a/lib/SP/Repositories/UserGroup/UserToUserGroupRepository.php b/lib/SP/Repositories/UserGroup/UserToUserGroupRepository.php
index 08dae784..34b0779a 100644
--- a/lib/SP/Repositories/UserGroup/UserToUserGroupRepository.php
+++ b/lib/SP/Repositories/UserGroup/UserToUserGroupRepository.php
@@ -62,7 +62,7 @@ class UserToUserGroupRepository extends Repository
      *
      * @param $userId
      *
-     * @return array
+     * @return \SP\Storage\Database\QueryResult
      * @throws \SP\Core\Exceptions\ConstraintException
      * @throws \SP\Core\Exceptions\QueryException
      */
@@ -72,7 +72,7 @@ class UserToUserGroupRepository extends Repository
         $queryData->setQuery('SELECT userGroupId FROM UserToUserGroup WHERE userId = ?');
         $queryData->addParam($userId);
 
-        return $this->db->doSelect($queryData)->getDataAsArray();
+        return $this->db->doSelect($queryData);
     }
 
     /**
@@ -81,7 +81,7 @@ class UserToUserGroupRepository extends Repository
      * @param int   $id
      * @param array $users
      *
-     * @return UserToUserGroupRepository
+     * @return int
      * @throws \SP\Core\Exceptions\ConstraintException
      * @throws \SP\Core\Exceptions\QueryException
      */
@@ -96,7 +96,7 @@ class UserToUserGroupRepository extends Repository
      *
      * @param $id int
      *
-     * @return $this
+     * @return int
      * @throws \SP\Core\Exceptions\ConstraintException
      * @throws \SP\Core\Exceptions\QueryException
      */
@@ -107,9 +107,7 @@ class UserToUserGroupRepository extends Repository
         $queryData->addParam($id);
         $queryData->setOnErrorMessage(__u('Error al eliminar los usuarios del grupo'));
 
-        $this->db->doQuery($queryData);
-
-        return $this;
+        return $this->db->doQuery($queryData)->getAffectedNumRows();
     }
 
     /**
@@ -118,12 +116,16 @@ class UserToUserGroupRepository extends Repository
      * @param int   $groupId
      * @param array $users
      *
-     * @return UserToUserGroupRepository
+     * @return int
      * @throws \SP\Core\Exceptions\ConstraintException
      * @throws \SP\Core\Exceptions\QueryException
      */
     public function add($groupId, array $users)
     {
+        if (empty($users)) {
+            return 0;
+        }
+
         $query = /** @lang SQL */
             'INSERT INTO UserToUserGroup (userId, userGroupId) VALUES ' . $this->getParamsFromArray($users, '(?,?)');
 
@@ -137,9 +139,7 @@ class UserToUserGroupRepository extends Repository
 
         $queryData->setOnErrorMessage(__u('Error al asignar los usuarios al grupo'));
 
-        $this->db->doQuery($queryData);
-
-        return $this;
+        return $this->db->doQuery($queryData)->getAffectedNumRows();
     }
 
     /**
@@ -147,7 +147,7 @@ class UserToUserGroupRepository extends Repository
      *
      * @param $id int
      *
-     * @return UserToUserGroupData[]
+     * @return \SP\Storage\Database\QueryResult
      * @throws \SP\Core\Exceptions\ConstraintException
      * @throws \SP\Core\Exceptions\QueryException
      */
@@ -158,6 +158,6 @@ class UserToUserGroupRepository extends Repository
         $queryData->setQuery('SELECT userGroupId, userId FROM UserToUserGroup WHERE userGroupId = ?');
         $queryData->addParam($id);
 
-        return $this->db->doSelect($queryData)->getDataAsArray();
+        return $this->db->doSelect($queryData);
     }
 }
\ No newline at end of file
diff --git a/lib/SP/Services/Upgrade/UpgradeDatabaseService.php b/lib/SP/Services/Upgrade/UpgradeDatabaseService.php
index 0a4fa68e..64f400a8 100644
--- a/lib/SP/Services/Upgrade/UpgradeDatabaseService.php
+++ b/lib/SP/Services/Upgrade/UpgradeDatabaseService.php
@@ -45,7 +45,7 @@ class UpgradeDatabaseService extends Service implements UpgradeInterface
     /**
      * @var array Versiones actualizables
      */
-    const UPGRADES = ['300.18010101', '300.18072302'];
+    const UPGRADES = ['300.18010101', '300.18072302', '300.18072501'];
 
     /**
      * @var Database
diff --git a/lib/SP/Services/UserGroup/UserToUserGroupService.php b/lib/SP/Services/UserGroup/UserToUserGroupService.php
index 5b0ed605..b30a590f 100644
--- a/lib/SP/Services/UserGroup/UserToUserGroupService.php
+++ b/lib/SP/Services/UserGroup/UserToUserGroupService.php
@@ -24,6 +24,8 @@
 
 namespace SP\Services\UserGroup;
 
+use SP\DataModel\UserToUserGroupData;
+use SP\Repositories\NoSuchItemException;
 use SP\Repositories\UserGroup\UserToUserGroupRepository;
 use SP\Services\Service;
 
@@ -43,19 +45,26 @@ class UserToUserGroupService extends Service
      * @param $id
      *
      * @return \SP\DataModel\UserToUserGroupData[]
+     * @throws NoSuchItemException
      * @throws \SP\Core\Exceptions\ConstraintException
      * @throws \SP\Core\Exceptions\QueryException
      */
     public function getById($id)
     {
-        return $this->userToUserGroupRepository->getById($id);
+        $result = $this->userToUserGroupRepository->getById($id);
+
+        if ($result->getNumRows() === 0) {
+            throw new NoSuchItemException(__u('Grupo no encontrado'), NoSuchItemException::INFO);
+        }
+
+        return $result->getDataAsArray();
     }
 
     /**
      * @param       $id
      * @param array $users
      *
-     * @return UserToUserGroupRepository
+     * @return int
      * @throws \SP\Core\Exceptions\ConstraintException
      * @throws \SP\Core\Exceptions\QueryException
      */
@@ -68,7 +77,7 @@ class UserToUserGroupService extends Service
      * @param int   $id
      * @param array $users
      *
-     * @return UserToUserGroupRepository
+     * @return int
      * @throws \SP\Core\Exceptions\ConstraintException
      * @throws \SP\Core\Exceptions\QueryException
      */
@@ -92,7 +101,8 @@ class UserToUserGroupService extends Service
     {
         $usersId = [];
 
-        foreach ($this->userToUserGroupRepository->getById($id) as $userToUserGroupData) {
+        /** @var UserToUserGroupData $userToUserGroupData */
+        foreach ($this->userToUserGroupRepository->getById($id)->getDataAsArray() as $userToUserGroupData) {
             $usersId[] = $userToUserGroupData->getUserId();
         }
 
@@ -125,7 +135,7 @@ class UserToUserGroupService extends Service
      */
     public function getGroupsForUser($userId)
     {
-        return $this->userToUserGroupRepository->getGroupsForUser($userId);
+        return $this->userToUserGroupRepository->getGroupsForUser($userId)->getDataAsArray();
     }
 
     /**
diff --git a/schemas/30018072501.sql b/schemas/30018072501.sql
new file mode 100644
index 00000000..dab85aed
--- /dev/null
+++ b/schemas/30018072501.sql
@@ -0,0 +1 @@
+ALTER TABLE `UserToUserGroup` ADD CONSTRAINT `uk_UserToUserGroup_01` UNIQUE (`userId`, `userGroupId`);
\ No newline at end of file
diff --git a/schemas/dbstructure.sql b/schemas/dbstructure.sql
index 718ec1a6..638edaec 100644
--- a/schemas/dbstructure.sql
+++ b/schemas/dbstructure.sql
@@ -519,6 +519,7 @@ CREATE TABLE `UserToUserGroup` (
   `userGroupId` smallint(5) unsigned NOT NULL,
   KEY `idx_UserToUserGroup_01` (`userId`),
   KEY `fk_UserToGroup_userGroupId` (`userGroupId`),
+  UNIQUE KEY `uk_UserToUserGroup_01` (`userId`, `userGroupId`),
   CONSTRAINT `fk_UserToGroup_userGroupId` FOREIGN KEY (`userGroupId`) REFERENCES `UserGroup` (`id`)
     ON DELETE CASCADE
     ON UPDATE CASCADE,
diff --git a/test/SP/Repositories/UserToUserGroupRepositoryTest.php b/test/SP/Repositories/UserToUserGroupRepositoryTest.php
new file mode 100644
index 00000000..2907cd5d
--- /dev/null
+++ b/test/SP/Repositories/UserToUserGroupRepositoryTest.php
@@ -0,0 +1,231 @@
+<?php
+/**
+ * sysPass
+ *
+ * @author    nuxsmin
+ * @link      https://syspass.org
+ * @copyright 2012-2018, Rubén Domínguez nuxsmin@$syspass.org
+ *
+ * This file is part of sysPass.
+ *
+ * sysPass is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * sysPass is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ *  along with sysPass.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+namespace SP\Tests\SP\Repositories;
+
+use SP\Core\Exceptions\ConstraintException;
+use SP\DataModel\UserToUserGroupData;
+use SP\Repositories\UserGroup\UserToUserGroupRepository;
+use SP\Storage\Database\DatabaseConnectionData;
+use SP\Test\DatabaseTestCase;
+use function SP\Test\setupContext;
+
+/**
+ * Class UserToUserGroupRepositoryTest
+ *
+ * @package SP\Tests\SP\Repositories
+ */
+class UserToUserGroupRepositoryTest extends DatabaseTestCase
+{
+    /**
+     * @var UserToUserGroupRepository
+     */
+    private static $repository;
+
+    /**
+     * @throws \DI\DependencyException
+     * @throws \DI\NotFoundException
+     * @throws \SP\Core\Context\ContextException
+     */
+    public static function setUpBeforeClass()
+    {
+        $dic = setupContext();
+
+        self::$dataset = 'syspass_userGroup.xml';
+
+        // Datos de conexión a la BBDD
+        self::$databaseConnectionData = $dic->get(DatabaseConnectionData::class);
+
+        // Inicializar el repositorio
+        self::$repository = $dic->get(UserToUserGroupRepository::class);
+    }
+
+    /**
+     * @throws \SP\Core\Exceptions\ConstraintException
+     * @throws \SP\Core\Exceptions\QueryException
+     */
+    public function testGetGroupsForUser()
+    {
+        $result = self::$repository->getGroupsForUser(3);
+
+        $this->assertEquals(1, $result->getNumRows());
+
+        $data = $result->getDataAsArray();
+
+        $this->assertCount(1, $data);
+        $this->assertEquals(2, $data[0]->userGroupId);
+
+        $result = self::$repository->getGroupsForUser(2);
+
+        $this->assertEquals(2, $result->getNumRows());
+
+        $data = $result->getDataAsArray();
+
+        $this->assertCount(2, $data);
+        $this->assertEquals(1, $data[0]->userGroupId);
+        $this->assertEquals(3, $data[1]->userGroupId);
+
+        $this->assertEquals(0, self::$repository->getGroupsForUser(10)->getNumRows());
+    }
+
+    /**
+     * @throws \SP\Core\Exceptions\ConstraintException
+     * @throws \SP\Core\Exceptions\QueryException
+     */
+    public function testUpdate()
+    {
+        $data = [3, 4];
+
+        self::$repository->update(1, $data);
+
+        $result = self::$repository->getById(1);
+
+        $this->assertEquals(2, $result->getNumRows());
+
+        /** @var UserToUserGroupData[] $data */
+        $data = $result->getDataAsArray();
+
+        $this->assertInstanceOf(UserToUserGroupData::class, $data[0]);
+        $this->assertEquals(1, $data[0]->getUserGroupId());
+        $this->assertEquals(3, $data[0]->getUserId());
+
+        $this->assertInstanceOf(UserToUserGroupData::class, $data[1]);
+        $this->assertEquals(1, $data[1]->getUserGroupId());
+        $this->assertEquals(4, $data[1]->getUserId());
+
+        $this->expectException(ConstraintException::class);
+
+        self::$repository->update(10, [3, 4]);
+    }
+
+    /**
+     * @throws ConstraintException
+     * @throws \SP\Core\Exceptions\QueryException
+     */
+    public function testGetById()
+    {
+        $result = self::$repository->getById(2);
+
+        $this->assertEquals(2, $result->getNumRows());
+
+        /** @var UserToUserGroupData[] $data */
+        $data = $result->getDataAsArray();
+
+        $this->assertCount(2, $data);
+
+        $this->assertInstanceOf(UserToUserGroupData::class, $data[0]);
+        $this->assertEquals(2, $data[0]->getUserGroupId());
+        $this->assertEquals(1, $data[0]->getUserId());
+
+        $this->assertInstanceOf(UserToUserGroupData::class, $data[1]);
+        $this->assertEquals(2, $data[1]->getUserGroupId());
+        $this->assertEquals(3, $data[1]->getUserId());
+
+        $data = self::$repository->getById(3)->getDataAsArray();
+
+        $this->assertCount(1, $data);
+
+        $this->assertInstanceOf(UserToUserGroupData::class, $data[0]);
+        $this->assertEquals(3, $data[0]->getUserGroupId());
+        $this->assertEquals(2, $data[0]->getUserId());
+
+        $this->assertEquals(0, self::$repository->getById(10)->getNumRows());
+    }
+
+    /**
+     * @throws ConstraintException
+     * @throws \SP\Core\Exceptions\QueryException
+     */
+    public function testDelete()
+    {
+        $this->assertEquals(1, self::$repository->delete(1));
+
+        $this->assertEquals(2, self::$repository->delete(2));
+
+        $this->assertEquals(0, self::$repository->delete(10));
+    }
+
+    /**
+     * @throws ConstraintException
+     * @throws \SP\Core\Exceptions\QueryException
+     */
+    public function testAdd()
+    {
+        $data = [3, 4];
+
+        self::$repository->add(1, $data);
+
+        $result = self::$repository->getById(1);
+
+        $this->assertEquals(3, $result->getNumRows());
+
+        /** @var UserToUserGroupData[] $data */
+        $data = $result->getDataAsArray();
+
+        $this->assertCount(3, $data);
+
+        $this->assertInstanceOf(UserToUserGroupData::class, $data[0]);
+
+        $this->assertEquals(1, $data[0]->getUserGroupId());
+        $this->assertEquals(2, $data[0]->getUserId());
+
+        $this->assertInstanceOf(UserToUserGroupData::class, $data[1]);
+        $this->assertEquals(1, $data[1]->getUserGroupId());
+        $this->assertEquals(3, $data[1]->getUserId());
+
+        $this->assertInstanceOf(UserToUserGroupData::class, $data[1]);
+        $this->assertEquals(1, $data[2]->getUserGroupId());
+        $this->assertEquals(4, $data[2]->getUserId());
+
+        $this->expectException(ConstraintException::class);
+
+        self::$repository->add(10, [3, 4]);
+    }
+
+    /**
+     * @throws \SP\Core\Exceptions\ConstraintException
+     * @throws \SP\Core\Exceptions\QueryException
+     */
+    public function testAddDuplicated()
+    {
+        $data = [2, 3, 4];
+
+        $this->expectException(ConstraintException::class);
+
+        self::$repository->add(1, $data);
+    }
+
+    /**
+     * @throws ConstraintException
+     * @throws \SP\Core\Exceptions\QueryException
+     */
+    public function testCheckUserInGroup()
+    {
+        $this->assertTrue(self::$repository->checkUserInGroup(1, 2));
+
+        $this->assertTrue(self::$repository->checkUserInGroup(2, 3));
+
+        $this->assertFalse(self::$repository->checkUserInGroup(3, 3));
+    }
+}
diff --git a/test/SP/Services/UserGroup/UserToUserGroupServiceTest.php b/test/SP/Services/UserGroup/UserToUserGroupServiceTest.php
new file mode 100644
index 00000000..4e43c341
--- /dev/null
+++ b/test/SP/Services/UserGroup/UserToUserGroupServiceTest.php
@@ -0,0 +1,192 @@
+<?php
+/**
+ * sysPass
+ *
+ * @author    nuxsmin
+ * @link      https://syspass.org
+ * @copyright 2012-2018, Rubén Domínguez nuxsmin@$syspass.org
+ *
+ * This file is part of sysPass.
+ *
+ * sysPass is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * sysPass is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ *  along with sysPass.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+namespace SP\Tests\SP\Services\UserGroup;
+
+use SP\Core\Exceptions\ConstraintException;
+use SP\DataModel\UserToUserGroupData;
+use SP\Repositories\NoSuchItemException;
+use SP\Services\UserGroup\UserToUserGroupService;
+use SP\Storage\Database\DatabaseConnectionData;
+use SP\Test\DatabaseTestCase;
+use function SP\Test\setupContext;
+
+/**
+ * Class UserToUserGroupServiceTest
+ *
+ * @package SP\Tests\SP\Services\UserGroup
+ */
+class UserToUserGroupServiceTest extends DatabaseTestCase
+{
+
+    /**
+     * @var UserToUserGroupService
+     */
+    private static $service;
+
+    /**
+     * @throws \DI\NotFoundException
+     * @throws \SP\Core\Context\ContextException
+     * @throws \DI\DependencyException
+     * @throws \SP\Core\Exceptions\SPException
+     */
+    public static function setUpBeforeClass()
+    {
+        $dic = setupContext();
+
+        self::$dataset = 'syspass_userGroup.xml';
+
+        // Datos de conexión a la BBDD
+        self::$databaseConnectionData = $dic->get(DatabaseConnectionData::class);
+
+        // Inicializar el servicio
+        self::$service = $dic->get(UserToUserGroupService::class);
+    }
+
+    /**
+     * @throws \SP\Core\Exceptions\ConstraintException
+     * @throws \SP\Core\Exceptions\QueryException
+     */
+    public function testCheckUserInGroup()
+    {
+        $this->assertTrue(self::$service->checkUserInGroup(1, 2));
+
+        $this->assertTrue(self::$service->checkUserInGroup(2, 3));
+
+        $this->assertFalse(self::$service->checkUserInGroup(3, 3));
+    }
+
+    /**
+     * @throws \SP\Core\Exceptions\ConstraintException
+     * @throws \SP\Core\Exceptions\QueryException
+     */
+    public function testGetGroupsForUser()
+    {
+        $data = self::$service->getGroupsForUser(3);
+
+        $this->assertCount(1, $data);
+        $this->assertEquals(2, $data[0]->userGroupId);
+
+        $data = self::$service->getGroupsForUser(2);
+
+        $this->assertCount(2, $data);
+        $this->assertEquals(1, $data[0]->userGroupId);
+        $this->assertEquals(3, $data[1]->userGroupId);
+
+        $data = self::$service->getGroupsForUser(10);
+
+        $this->assertCount(0, $data);
+    }
+
+    /**
+     * @throws \SP\Core\Exceptions\ConstraintException
+     * @throws \SP\Core\Exceptions\QueryException
+     */
+    public function testAdd()
+    {
+        $data = [3, 4];
+
+        self::$service->add(1, $data);
+
+        $this->assertEquals([2, 3, 4], self::$service->getUsersByGroupId(1));
+
+        $this->expectException(ConstraintException::class);
+
+        self::$service->add(10, $data);
+    }
+
+    /**
+     * @throws \SP\Core\Exceptions\ConstraintException
+     * @throws \SP\Core\Exceptions\QueryException
+     */
+    public function testAddDuplicated()
+    {
+        $data = [2, 3, 4];
+
+        $this->expectException(ConstraintException::class);
+
+        self::$service->add(1, $data);
+    }
+
+    /**
+     * @throws ConstraintException
+     * @throws \SP\Core\Exceptions\QueryException
+     */
+    public function testUpdate()
+    {
+        $data = [3, 4];
+
+        self::$service->update(1, $data);
+
+        $this->assertEquals($data, self::$service->getUsersByGroupId(1));
+
+        $this->expectException(ConstraintException::class);
+
+        self::$service->update(10, $data);
+    }
+
+    /**
+     * @throws ConstraintException
+     * @throws \SP\Core\Exceptions\QueryException
+     * @throws \SP\Repositories\NoSuchItemException
+     */
+    public function testGetById()
+    {
+        $data = self::$service->getById(2);
+
+        $this->assertCount(2, $data);
+
+        $this->assertInstanceOf(UserToUserGroupData::class, $data[0]);
+
+        $this->assertEquals(2, $data[0]->getUserGroupId());
+        $this->assertEquals(1, $data[0]->getUserId());
+
+        $this->assertEquals(2, $data[1]->getUserGroupId());
+        $this->assertEquals(3, $data[1]->getUserId());
+
+        $data = self::$service->getById(3);
+
+        $this->assertCount(1, $data);
+
+        $this->assertEquals(3, $data[0]->getUserGroupId());
+        $this->assertEquals(2, $data[0]->getUserId());
+
+        $this->expectException(NoSuchItemException::class);
+
+        self::$service->getById(10);
+    }
+
+    /**
+     * @throws ConstraintException
+     * @throws \SP\Core\Exceptions\QueryException
+     */
+    public function testGetUsersByGroupId()
+    {
+        $data = self::$service->getUsersByGroupId(2);
+
+        $this->assertCount(2, $data);
+
+        $this->assertEquals([1, 3], $data);
+    }
+}
diff --git a/test/res/datasets/syspass_userGroup.xml b/test/res/datasets/syspass_userGroup.xml
index 420c837f..207598ba 100644
--- a/test/res/datasets/syspass_userGroup.xml
+++ b/test/res/datasets/syspass_userGroup.xml
@@ -81,6 +81,10 @@
             <field name="userId">2</field>
             <field name="userGroupId">1</field>
         </row>
+        <row>
+            <field name="userId">2</field>
+            <field name="userGroupId">3</field>
+        </row>
     </table_data>
 </database>
 </mysqldump>
\ No newline at end of file