Forget that...my calculation will NOT work!
The piece of the formula that should check for duplicates (IF(FREQUENCY(MATCH(PrtNbr,PrtNbr,0),MATCH(PrtNbr,PrtNbr,0))>0,1)) is not correct.
If I apply the sum like this:
SUM(IF(FREQUENCY(MATCH(PrtNbr,PrtNbr,0),MATCH(PrtNbr,PrtNbr,0))>0,1)) then I...