Moved implementation of check for existance of username on creating user from user service to repository.

src/Umbraco.Infrastructure/Persistence/Repositories/Implement/UserRepository.cs

@@ -626,6 +626,11 @@ ORDER BY colName";
         }
 
         public bool Exists(string username)
+        {
+            return ExistsByUserName(username);
+        }
+
+        public bool ExistsByUserName(string username)
         {
             var sql = SqlContext.Sql()
                 .SelectCount()
@@ -635,6 +640,16 @@ ORDER BY colName";
             return Database.ExecuteScalar<int>(sql) > 0;
         }
 
+        public bool ExistsByLogin(string login)
+        {
+            var sql = SqlContext.Sql()
+                .SelectCount()
+                .From<UserDto>()
+                .Where<UserDto>(x => x.Login == login);
+
+            return Database.ExecuteScalar<int>(sql) > 0;
+        }
+
         /// <summary>
         /// Gets a list of <see cref="IUser"/> objects associated with a given group
         /// </summary>