This is basically a reformulation of the Birthday Paradox. Even with a large variety of makes and models of luggage (cf. 365+ days in a year), the chances of any two being the same increases superlinearly wrt passengers.
That's what I had in mind. I figure on average half the passengers on any given flight are returning home. People living in the same area tend to have the same retail choices. The number of variations among this group probably isn't in the hundreds.
My point is that the variety of luggage is so vast (i'd hazard tens of thousands of models) that the probability would be low even for a quite high number of passengers.