Calc based on # of times duplicated

[Guest]

Is there a function that will find duplicate records and
then if it is duplicate it will then perform a calculation?
And where would I put this function?

Example:
If Invoice ## is listed more than once, divide subtotal by
# of times listed.

I never used Access before yesterday, but I have written
several queries based on date range and another based on
If statements and they work perfectly. The example above
has me stumped.

Thanks

Lisa