Duplicate permalinks

The version with the p is how it works when you don’t use permalinks. It continues to work even after you set up permalinks. It just tells the page what post id to load.

As oriecat said, both are valid, but If you want to be very strict about it, you could enable this plugin: https://scott.yang.id.au/2005/05/permalink-redirect/

Thanks for the pointer macmanx! It looks like that plugin solved the problem. Nifty.