string source = "12234";
            Regex reg = new Regex(@"([\d])(?=\1)");
            source = reg.Replace(source, ""); //source = 1234

------解决方案--------------------

linq
C# code

 string tempStr = "12234";
                string temp_list=string.Join("",tempStr.ToCharArray().Distinct());//1234

------解决方案--------------------

Try
C# code

string source = "122234";
                Regex reg = new Regex(@"(\d)(?=[^\1]*\1)");
                source = reg.Replace(source, ""); //source = 1234