Sql Server merge replication primary key violations
I'd change the IDENTITY so one is odd, one even. Or negative/positive.

Probably the simplest way to do it...

Edit: I assumed identity PK columns.

I'd suggest you update the question with the relevant info contained in the comment. It's incomplete otherwise.

You said "The desired behaviour is that after the merge all rows are identical. The first one to the publisher wins..."

This sounds like conflict resolution, which is how you'd deal with PK collisions.