First I would create a new calculation that returns a generic month and year for reporting such a:
cMonthYear = Date ( Month ( SalesDate ); 1; Year ( SalesDate ) )
Now create a self join keyed on the fkCustomerID = fkCustomerID AND cMonthYear = cMonthYear.
You can also create another calculation to figure out how many records a customer has for a particular month.
cMonthYearCount = Count ( SelfJoin::pkInvoiceID )
Now you can do a find for the current month in the SalesDate field for anything > 1 in cMonthYearCount.
Here's a hint to get you started:
If you enter into find mode and place a ! by itself in a field. You will be able to find all records that contain duplicate entries in that field. Sort on that field and you will have successfully grouped the records by the duplicate values.
You can create subsummary reports without any "body" part. Your sub-summary can then hold summary fields that do aggregate/statistical functions to produce counts, averages, etc.
Thanks for the help! This works great! Now is there a way to use a summary or calculation field to tell me the specific number of repeat customers I have?
Well one way to do it is after you find all the invoice records that match your criteria, you can use a Go to Related Records back to customers table ( show related only, match found set ).