you can do a search operation with the datatable and get to know whether there is an entry for the particular value or not. For checking that you can follow following code:
DataRow found = Obj_DT_SRDF.Select("ls_DRName = '" + ls_DRName + "'");
if(found.Length == 0)
// Insert the record or else don't insert
Hope this will help you out in checking the duplicate values and preventing the insertion of the duplicate values.