string test_str = "abcabc12345612ab";
                test_str = string.Join("", test_str.ToArray().Distinct());//abc123456

------解决方案--------------------
C# code

void Main()
{
    string str="abcabc12345612ab";
 
Regex reg = new Regex(@"((\w)\w*?)\2");

while (str != (str = reg.Replace(str, "$1"))) { }

Console.WriteLine(str);  //abc123456

}
//不过我还是建议你用LINQ来实现，这不是正则干的活~~~

------解决方案--------------------
C# code
        Regex regex = new Regex(@"(\w)(?=.*?\1)");
        string str = "abcabc12345612ab";
        Response.Write(regex.Replace(str,""));

------解决方案--------------------
C# code

 string str="abcabc12345612ab";
 str=new string(str.ToCharArray().Distinct().ToArray());

------解决方案--------------------
支持一楼