tag a record

[Steven Waters]

G-day,
I am writing a small program that I want
to tag with a "X" duplicate P/N's in a file.
I can't figure out how to get the first instance
of a duplicate P/N. For example if there are 3 records
with the same P/N I only get a "X" by record 2 and 3. record 1
is not X'd.

  1  -------   -   -   
        If: 1 eq ab    
       Then: 12="X";end 
  2  -------   -   -   -
        If: 1 ne ab    
       Then: ab(40,,g)=1    

Thanks in advance!
Steve