Trying to detect and stop a duplicate record from being inserted on customer record

So I have written this server side script to detect when a user is tring to insert a duplicate record based on userid already in the custom record table.

So when I exit with a FALSE return code the record is still inserted into the custom record table.. How do I prevent the save from inserting a duplicate record... This script is called on "before submit entry point" when the custom record is being modifiied.

Thanks

Tom

function Chk_Dup_Dialup_ID(request, response)
{
   try
   {
      // Invoke Server SuiteScript code here while generously
      // sprinkling debug statements by calling response.writeLine( output ) */