Formula Below:

I am looking up a product number (Cell $A416 in formula below) and returning
its corresponding inventory count.

The problem I have is the product number may appear more than once in the
lookup range. My formula is only giving the first occurance - its not adding
the count for all instances.
Incidently, I get back alot on NA's, that is why I have the ISNA part.

Any help?

IF(ISNA(VLOOKUP($A416,'I631_2
sales'!$A$10:$D2671,4,FALSE)),"0",(VLOOKUP($A416,' I631_2
sales'!$A$10:$D2671,4,FALSE)))