That's because I've only told you part of the story. This is part of a converting project. I'm moving all of the data from one database to another, but I need to keep some sort of track of history in the source database. Those systems are not equal so I have to align metadata from source to destination system. When the data is transferred to the final databases I merge column A and C in table B into the same column and then there is only one PK.

I could probably solve this otherwise but that's how it works for me.