public class StringDataStructure  
    {  
        //Find the duplicate character  
        public StringBuilder GetDuplicateCharacter(string _title)  
        {  
            StringBuilder result = new StringBuilder();  
            StringBuilder duplicateChar = new StringBuilder();  
  
            foreach (var item in _title)  
            {  
                if (result.ToString().IndexOf(item.ToString().ToLower()) == -1)  
                {  
                    result.Append(item);  
                }  
                else  
                {  
                    duplicateChar.Append(item);  
                }  
            }  
  
            return duplicateChar;  
        }  
   }