关键是使用StringCollection，需要引入System.Collections.Specialized命名空间。

代码如下：

using System.Text;
using System.Collections.Specialized;  
          
//去除重复的数据
StringBuilder sb = new StringBuilder();
sb.Append(1+",");
sb.Append(2+",");
sb.Append(3+",");
sb.Append(1+",");
sb.Append(3+",");
StringCollection sc = new StringCollection();
string[] arr = sb.ToString().TrimEnd(',').Split(',');
foreach (string str in arr)
{
    if (!sc.Contains(str))
    {
         sc.Add(str);
    }
}
StringBuilder sb2 = new StringBuilder();
foreach (string str in sc)
{
    sb2.Append(str + ",");
}
return sb2.ToString().TrimEnd(',');