app/user: fix update user error when an e-mail address already in use (#16269)

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/user_test.go

@@ -461,7 +461,8 @@ func TestUpdateUserEmail(t *testing.T) {
 
 		user.Email = newEmail
 		user3, err := th.App.UpdateUser(user, false)
-		assert.NotNil(t, err)
+		require.NotNil(t, err)
+		assert.Equal(t, err.Id, "store.sql_user.update.email_taken.app_error")
 		assert.Nil(t, user3)
 	})
 
@@ -492,6 +493,21 @@ func TestUpdateUserEmail(t *testing.T) {
 		assert.Nil(t, err)
 		assert.Equal(t, botuser2.Email, newBotEmail)
 	})
+
+	t.Run("NoVerificationAlreadyUsedEmail", func(t *testing.T) {
+		th.App.UpdateConfig(func(cfg *model.Config) {
+			*cfg.EmailSettings.RequireEmailVerification = false
+		})
+
+		user2 := th.CreateUser()
+		newEmail := user2.Email
+
+		user.Email = newEmail
+		user3, err := th.App.UpdateUser(user, false)
+		require.NotNil(t, err)
+		assert.Equal(t, err.Id, "store.sql_user.update.email_taken.app_error")
+		assert.Nil(t, user3)
+	})
 }
 
 func getUserFromDB(a *App, id string, t *testing.T) *model.User {