This reminds me of what happened to me: I had an IDB several years ago, connecting YYZ-YUL from HKG. I had boarded the plane and sat down (I recall the seat being 12A), when a passenger walked up to me with a boarding pass for the same seat. The FA checked out the problem and it turned out that my seat had been given to the other passenger because my checked luggage wasn't able to make the transfer onto the YYZ-YUL flight in time, so I was put on a later flight. I didn't receive any compensation, but it was a Rapidair flight so I didn't mind much.