问如何在使用list AddRange时删除重复值？C#

EN

 pmlist = [0] {@DeptCode} [1] {@DeptCode} [2] {@DeptName}  
 [3] {@ParentDeptName} [4] {@ParentDeptCode} [5] {@DeptLevel}

//table : ShadowDeptTable ,  keys = {@DeptCode}, 
//pms =   [0] {@DeptCode} [1] {@DeptName} [2] {@ParentDeptName}  
//        [3] {@ParentDeptCode} [4] {@DeptLevel}    

public static int InsertOrUpdate(string table, SqlParameter[] keys, params SqlParameter[] pms)
{
   int st = 0;
   string constr = Util.GetPropVal(Words.PropConnectionString);
   var obj = SqlHelper.ExecuteScalar(constr,CommandType.Text,sb.ToString(),keys);
   StringBuilder sb=new StringBuilder();
   sb = sb.Append("insert into " + table + "(");
   //columns
   string cols = null;
   //values
   string vals = null;
   List<SqlParameter> pmlist = new List<SqlParameter>(keys);
   pmlist.AddRange(pms);
   cols = string.Join(",", pmlist.Select(a => "["+a.ParameterName.Substring(1, a.ParameterName.Length-1)+"]"));
   vals = string.Join(",", pmlist.Select(a => "'" + a.Value + "'"));
   sb = sb.Append(cols);
   sb = sb.Append(") values(");
   sb = sb.Append(vals);
   sb = sb.Append(")");
   obj = SqlHelper.ExecuteScalar(constr, CommandType.Text, sb.ToString());
   return Convert.ToInt32(obj);
}