Is this a one-time check?
Is there only one program doing the update?
If it is a one time check, you can do a summary sort and get counts by whatever field(s) you are using to check for uniqueness.
Else, create an LF with your key.
Write a routine that will check the file after the update. This routine can either do an SQL COUNT(*) or CHAIN & READE, to see how many records have the value you just had in the record that was written/updated.
Now if there is only one program doing the update, insert this routine in that program.
If there are multiple programs doing the update, create a trigger program with this routine.