Return the value of a duplicate

I have a spreadsheet that shows a location number, merchant ID and tandem ID. Some location numbers are duplicate and can have multiple merchant ID's (which are unique). Tandem ID's are unique to a location number.

In my spreadsheet, the duplicate location numbers only have a tandem ID for one of the merchant ID's. For example...

location # merchant ID tandem ID
12345 55555 AAAA
12345 66666
12345 77777

How can a write a formula to fill out the 'AAAA' tandem Id for all duplicate locations. The logic would be if tandem id is blank and location number is more than one, look in an array of the duplicate location numbers and return the corresponding tandem ID.

The huge problem is sometimes the location number with the tandem ID may be the first, middle or last of the duplicates, so I can't just say count up one. Is this possible????

Thanks!!