Filtering Duplicates

I am trying to make a spreadsheet that will say “LATE” at the end of each column if the ship date is >17 days after the order date. The difficulty is that a purchase order is only considered late once, so it will need to look at the “PO” column and only return “LATE” once regardless of how many products shipped from this Po. Below is an example where only OrdNbr’s 2 and 6 should show as late. I do not want to use “remove duplicates” because I want the total number of orders to show also. I can easily do IF(ShipDate-OrderDate>17, “LATE”, “”) but how can I filter the duplicate PO’s without removing duplicates? Thanks for your help and time!

OrdNbr PO OrdDate Ship Date Product#
1 1251 9/1/2010 9/2/2010 A110
2 1251 9/1/2010 9/20/2010 A111
3 1251 9/1/2010 9/4/2010 A112
4 551 9/2/2010 9/20/2010 A111
5 551 9/2/2010 9/5/2010 A112
6 551 9/2/2010 9/20/2010 A113