Duplicates in Multiple Records

[endonservices]

I want to locate all invoice records which contain a duplicate entry in a field "Customer Ref" and then have only ONE entry of each duplicate show up in a found set .... for subsequent processing my my own simple direct mail engine (FMpro Server v11.3)

Is there a short cut to achieving this please.......?

I just want to send a "special offers" letter to all the customers who have more than one invoice present on the invoice transaction file.

My probelm is probably my own relatively sloppy understanding of the relational model ...... but I would greatly appreciate a wizardly point in the right direction.....

[MetaSys Software]

Each Customer can have one or multiple invoices present in invoice transaction file (table). In Customer table, create one calculation field, which gives the count of invoices for particular customer. Search customer who have invoice count greater than 1 and you can send an email (special offer letter) to the found set.