Was travelling yesterday in a party of 2 from BOS to YYZ in J, and found another passenger had a boarding pass with the same seat assignment as me.

Fortunately, there was 1 empty seat in the J cabin, so the FA worked everything out. But I'm curious what would have happened if the cabin was full?

If they had to bump someone down to Y or off the flight completely, is there a published formula for the compensation calculation or a list of passenger options/rights? I was flying in paid business, but with it being a P fare, could AC try to offer something stupid like the difference between what I paid 6 weeks ago and what full Y was selling for on the day of travel?