I'm wrestling with a search that seems like it should be easier than it it is...  but I'm likely overcomplicating it.

I have Invoices that have a custom field on them {cust_thing}.

I want to look at ALL Invoices in the system for the first UNIQUE occurrence of invoices with that field within a date range.

Let me say it this way...  IF - over a given time frame (let's say last 1 month) - there is an Invoice created where {cust_thing} is not empty AND it's the FIRST occurrence of an invoice in the system with {cust_thing} populated with that unique value - I want to return that Invoice in the search results.  If cust_thing is populated BUT it's not the FIRST record (based on date) that has that has the same value in cust_thing, I don't want it in the results.