diff --git a/integrations/signin_test.go b/integrations/signin_test.go
index 3ea8866150..a6e4b7d4d2 100644
--- a/integrations/signin_test.go
+++ b/integrations/signin_test.go
@@ -51,8 +51,6 @@ func TestSignin(t *testing.T) {
 		{username: "wrongUsername", password: "password", message: i18n.Tr("en", "form.username_password_incorrect")},
 		{username: "user15", password: "wrongPassword", message: i18n.Tr("en", "form.username_password_incorrect")},
 		{username: "user1@example.com", password: "wrongPassword", message: i18n.Tr("en", "form.username_password_incorrect")},
-		// test for duplicate email
-		{username: "user2@example.com", password: "password", message: i18n.Tr("en", "form.email_been_used")},
 	}
 
 	for _, s := range samples {
diff --git a/models/user/user_test.go b/models/user/user_test.go
index 70591c8c12..a5f47172ee 100644
--- a/models/user/user_test.go
+++ b/models/user/user_test.go
@@ -235,6 +235,20 @@ func TestCreateUserInvalidEmail(t *testing.T) {
 	assert.True(t, IsErrEmailInvalid(err))
 }
 
+func TestCreateUserEmailAlreadyUsed(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
+	user := unittest.AssertExistsAndLoadBean(t, &User{ID: 2}).(*User)
+
+	// add new user with user2's email
+	user.Name = "testuser"
+	user.LowerName = strings.ToLower(user.Name)
+	user.ID = 0
+	err := CreateUser(user)
+	assert.Error(t, err)
+	assert.True(t, IsErrEmailAlreadyUsed(err))
+}
+
 func TestGetUserIDsByNames(t *testing.T) {
 	assert.NoError(t, unittest.PrepareTestDatabase())
 
diff --git a/services/auth/signin.go b/services/auth/signin.go
index 4392e861f9..a6f2d14b5c 100644
--- a/services/auth/signin.go
+++ b/services/auth/signin.go
@@ -24,17 +24,18 @@ import (
 func UserSignIn(username, password string) (*user_model.User, *auth.Source, error) {
 	var user *user_model.User
 	if strings.Contains(username, "@") {
-		user = &user_model.User{Email: strings.ToLower(strings.TrimSpace(username))}
+		emailAddress := user_model.EmailAddress{LowerEmail: strings.ToLower(strings.TrimSpace(username))}
 		// check same email
-		cnt, err := db.Count(user)
+		has, err := db.GetEngine(db.DefaultContext).Where("is_activated=?", true).Get(&emailAddress)
 		if err != nil {
 			return nil, nil, err
 		}
-		if cnt > 1 {
-			return nil, nil, user_model.ErrEmailAlreadyUsed{
+		if !has {
+			return nil, nil, user_model.ErrEmailAddressNotExist{
 				Email: user.Email,
 			}
 		}
+		user = &user_model.User{ID: emailAddress.UID}
 	} else {
 		trimmedUsername := strings.TrimSpace(username)
 		if len(trimmedUsername) == 0 {
diff --git a/services/auth/source/ldap/source_authenticate.go b/services/auth/source/ldap/source_authenticate.go
index 52971bb87e..bc35bc20f6 100644
--- a/services/auth/source/ldap/source_authenticate.go
+++ b/services/auth/source/ldap/source_authenticate.go
@@ -19,10 +19,14 @@ import (
 // Authenticate queries if login/password is valid against the LDAP directory pool,
 // and create a local user if success when enabled.
 func (source *Source) Authenticate(user *user_model.User, userName, password string) (*user_model.User, error) {
-	sr := source.SearchEntry(userName, password, source.authSource.Type == auth.DLDAP)
+	loginName := userName
+	if user != nil {
+		loginName = user.LoginName
+	}
+	sr := source.SearchEntry(loginName, password, source.authSource.Type == auth.DLDAP)
 	if sr == nil {
 		// User not in LDAP, do nothing
-		return nil, user_model.ErrUserNotExist{Name: userName}
+		return nil, user_model.ErrUserNotExist{Name: loginName}
 	}
 
 	isAttributeSSHPublicKeySet := len(strings.TrimSpace(source.AttributeSSHPublicKey)) > 0