diff --git a/server/common/dao/src/main/java/org/kaaproject/kaa/server/common/dao/service/EndpointServiceImpl.java b/server/common/dao/src/main/java/org/kaaproject/kaa/server/common/dao/service/EndpointServiceImpl.java
index b51e586640..3f37b1bef3 100644
--- a/server/common/dao/src/main/java/org/kaaproject/kaa/server/common/dao/service/EndpointServiceImpl.java
+++ b/server/common/dao/src/main/java/org/kaaproject/kaa/server/common/dao/service/EndpointServiceImpl.java
@@ -74,6 +74,7 @@
 
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Collection;
 import java.util.Collections;
 import java.util.List;
 
@@ -385,36 +386,44 @@ public EndpointProfileDto saveEndpointProfile(EndpointProfileDto endpointProfile
   }
 
   @Override
-  public EndpointProfileDto attachEndpointToUser(String userExternalId, String tenantId,
-                                                 EndpointProfileDto profile) {
-    validateString(userExternalId, "Incorrect userExternalId "
-                                   + userExternalId);
-    EndpointUser endpointUser = endpointUserDao.findByExternalIdAndTenantId(userExternalId,
-            tenantId);
-    if (endpointUser
-        == null) {
-      LOG.info("Creating new endpoint user with external id: [{}] in context of [{}] tenant",
-              userExternalId, tenantId);
+  public EndpointProfileDto attachEndpointToUser(String userExternalId, String tenantId, EndpointProfileDto profile) {
+    validateString(userExternalId, "Incorrect userExternalId " + userExternalId);
+    EndpointUser endpointUser = endpointUserDao.findByExternalIdAndTenantId(userExternalId, tenantId);
+    String currentEndpointUserId = profile.getEndpointUserId();
+    String endpointId = profile.getId();
+    if (endpointUser == null) {
+      LOG.info("Creating new endpoint user with external id: [{}] in context of [{}] tenant", userExternalId, tenantId);
       EndpointUserDto endpointUserDto = new EndpointUserDto();
       endpointUserDto.setTenantId(tenantId);
       endpointUserDto.setExternalId(userExternalId);
       endpointUserDto.setUsername(userExternalId);
       endpointUser = endpointUserDao.save(endpointUserDto);
     }
+    
     List<String> endpointIds = endpointUser.getEndpointIds();
-    if (endpointIds
-        == null) {
+    if (endpointIds == null) {
       endpointIds = new ArrayList<>();
       endpointUser.setEndpointIds(endpointIds);
-    } else if (endpointIds
-               != null
-               && endpointIds.contains(profile.getId())) {
-      LOG.warn("Endpoint is already assigned to current user {}.", profile.getEndpointUserId());
+    } else if (endpointIds.contains(endpointId)) {
+      LOG.warn("Endpoint is already assigned to current user {}.", currentEndpointUserId);
       return profile;
     }
-    endpointIds.add(profile.getId());
+    endpointIds.add(endpointId);
+    
+    if (currentEndpointUserId != null) {
+      // Remove the endpoint ID from current user collection
+      EndpointUser currentEndpointUser = endpointUserDao.findById(currentEndpointUserId);
+      if (currentEndpointUser != null) {
+        Collection<String> currentUserEndpointIds = currentEndpointUser.getEndpointIds();
+        if (currentUserEndpointIds != null && currentUserEndpointIds.remove(endpointId)) {
+          endpointUserDao.save(currentEndpointUser);
+        }
+      }
+    }
+    
     endpointUser = endpointUserDao.save(endpointUser);
-    profile.setEndpointUserId(endpointUser.getId());
+    String newEndpointUserId = endpointUser.getId();
+    profile.setEndpointUserId(newEndpointUserId);
     while (true) {
       try {
         LOG.trace("Save endpoint user {} and endpoint profile {}", endpointUser, profile);
@@ -423,7 +432,7 @@ public EndpointProfileDto attachEndpointToUser(String userExternalId, String ten
         LOG.warn("Optimistic lock detected in endpoint profile ", Arrays.toString(profile
                 .getEndpointKey()), ex);
         profile = findEndpointProfileByKeyHash(profile.getEndpointKeyHash());
-        profile.setEndpointUserId(endpointUser.getId());
+        profile.setEndpointUserId(newEndpointUserId);
       }
     }
   }
diff --git a/server/common/dao/src/test/java/org/kaaproject/kaa/server/common/dao/service/EndpointServiceImplTest.java b/server/common/dao/src/test/java/org/kaaproject/kaa/server/common/dao/service/EndpointServiceImplTest.java
index 2495d1106e..313817bc68 100644
--- a/server/common/dao/src/test/java/org/kaaproject/kaa/server/common/dao/service/EndpointServiceImplTest.java
+++ b/server/common/dao/src/test/java/org/kaaproject/kaa/server/common/dao/service/EndpointServiceImplTest.java
@@ -36,6 +36,7 @@
 import org.kaaproject.kaa.server.common.dao.exception.KaaOptimisticLockingFailureException;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
+import org.springframework.util.CollectionUtils;
 
 import java.util.ArrayList;
 import java.util.Arrays;
@@ -43,6 +44,7 @@
 import java.util.Comparator;
 import java.util.Iterator;
 import java.util.List;
+import java.util.UUID;
 import java.util.concurrent.Callable;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.ExecutorService;
@@ -264,6 +266,107 @@ public void attachEndpointToUserTest() {
     Assert.assertEquals(1, endpointIds.size());
     Assert.assertEquals(endpointProfileDto.getId(), endpointIds.get(0));
   }
+  
+
+  @Test
+  public void attachEndpointToNewUserByExternalIdTest() {
+    TenantDto tenant = generateTenantDto();
+    String endpointGroupId = "124";
+    EndpointProfileDto endpointProfile = generateEndpointProfileWithGroupIdDto(endpointGroupId);
+    String userExternalId = UUID.randomUUID().toString();
+    String tenantId = tenant.getId();
+
+    EndpointProfileDto savedEndpointProfile = endpointService.attachEndpointToUser(userExternalId, tenantId, endpointProfile);
+
+    Assert.assertNotNull(savedEndpointProfile);
+    Assert.assertNotNull(savedEndpointProfile.getEndpointUserId());
+    EndpointUserDto attachedEndpointUser = endpointService.findEndpointUserById(savedEndpointProfile.getEndpointUserId());
+    Assert.assertNotNull(attachedEndpointUser);
+    Assert.assertEquals(userExternalId, attachedEndpointUser.getExternalId());
+    Assert.assertEquals(userExternalId, attachedEndpointUser.getUsername());
+    Assert.assertEquals(tenantId, attachedEndpointUser.getTenantId());
+    List<String> endpointIds = attachedEndpointUser.getEndpointIds();
+    Assert.assertNotNull(endpointIds);
+    Assert.assertEquals(1, endpointIds.size());
+    Assert.assertEquals(endpointProfile.getId(), endpointIds.get(0));
+  }
+
+  @Test
+  public void attachEndpointToExistingUserByExternalIdTest() {
+    TenantDto tenant = generateTenantDto();
+    String endpointGroupId = "124";
+    EndpointProfileDto endpointProfile = generateEndpointProfileWithGroupIdDto(endpointGroupId);
+    String tenantId = tenant.getId();
+    EndpointUserDto endpointUserDto = generateEndpointUserDto(tenantId);
+    String userExternalId = endpointUserDto.getExternalId();
+
+    EndpointProfileDto savedEndpointProfile = endpointService.attachEndpointToUser(userExternalId, tenantId, endpointProfile);
+
+    Assert.assertNotNull(savedEndpointProfile);
+    Assert.assertNotNull(savedEndpointProfile.getEndpointUserId());
+    EndpointUserDto attachedEndpointUser = endpointService.findEndpointUserById(savedEndpointProfile.getEndpointUserId());
+    Assert.assertNotNull(attachedEndpointUser);
+    Assert.assertEquals(userExternalId, attachedEndpointUser.getExternalId());
+    Assert.assertEquals(tenantId, attachedEndpointUser.getTenantId());
+    List<String> endpointIds = attachedEndpointUser.getEndpointIds();
+    Assert.assertNotNull(endpointIds);
+    Assert.assertEquals(1, endpointIds.size());
+    Assert.assertEquals(endpointProfile.getId(), endpointIds.get(0));
+  }
+
+  @Test
+  public void attachEndpointToAlreadyAttachedUserByExternalIdTest() {
+    TenantDto tenant = generateTenantDto();
+    String endpointGroupId = "124";
+    EndpointProfileDto endpointProfile = generateEndpointProfileWithGroupIdDto(endpointGroupId);
+    String tenantId = tenant.getId();
+    EndpointUserDto endpointUserDto = generateEndpointUserDto(tenantId);
+    String userExternalId = endpointUserDto.getExternalId();
+
+    EndpointProfileDto savedEndpointProfile = endpointService.attachEndpointToUser(userExternalId, tenantId, endpointProfile);
+    savedEndpointProfile = endpointService.attachEndpointToUser(userExternalId, tenantId, savedEndpointProfile);
+
+    Assert.assertNotNull(savedEndpointProfile);
+    Assert.assertNotNull(savedEndpointProfile.getEndpointUserId());
+    EndpointUserDto attachedEndpointUser = endpointService.findEndpointUserById(savedEndpointProfile.getEndpointUserId());
+    Assert.assertNotNull(attachedEndpointUser);
+    Assert.assertEquals(userExternalId, attachedEndpointUser.getExternalId());
+    Assert.assertEquals(tenantId, attachedEndpointUser.getTenantId());
+    List<String> endpointIds = attachedEndpointUser.getEndpointIds();
+    Assert.assertNotNull(endpointIds);
+    Assert.assertEquals(1, endpointIds.size());
+    Assert.assertEquals(endpointProfile.getId(), endpointIds.get(0));
+  }
+
+  @Test
+  public void attachEndpointAlreadyAttachedToAnotherUserByExternalIdTest() {
+    TenantDto tenant = generateTenantDto();
+    String endpointGroupId = "124";
+    EndpointProfileDto endpointProfile = generateEndpointProfileWithGroupIdDto(endpointGroupId);
+    String tenantId = tenant.getId();
+    EndpointUserDto endpointUser1 = generateEndpointUserDto(tenantId);
+    String userExternalId1 = endpointUser1.getExternalId();
+
+    EndpointUserDto endpointUser2 = generateEndpointUserDto(tenantId);
+    String userExternalId2 = endpointUser2.getExternalId();
+
+
+    EndpointProfileDto savedEndpointProfile = endpointService.attachEndpointToUser(userExternalId1, tenantId, endpointProfile);
+    savedEndpointProfile = endpointService.attachEndpointToUser(userExternalId2, tenantId, savedEndpointProfile);
+
+    EndpointUserDto attachedUser = endpointService.findEndpointUserById(savedEndpointProfile.getEndpointUserId());
+    endpointUser1 = endpointService.findEndpointUserByExternalIdAndTenantId(userExternalId1, tenantId);
+    endpointUser2 = endpointService.findEndpointUserByExternalIdAndTenantId(userExternalId2, tenantId);
+    Assert.assertNotNull(attachedUser);
+    Assert.assertEquals(endpointUser2, attachedUser);
+    List<String> user1EndpointIds = endpointUser1.getEndpointIds();
+    List<String> user2EndpointIds = endpointUser2.getEndpointIds();
+    Assert.assertTrue(CollectionUtils.isEmpty(user1EndpointIds));
+    Assert.assertFalse(CollectionUtils.isEmpty(user2EndpointIds));
+    Assert.assertEquals(1, user2EndpointIds.size());
+    Assert.assertEquals(endpointProfile.getId(), user2EndpointIds.get(0));
+    Assert.assertEquals(endpointUser2.getId(), savedEndpointProfile.getEndpointUserId());
+  }
 
   @Test
   public void findEndpointProfilesByExternalUserIdTest() {