Originally Posted by yyznomad

As per the others, you must clear Canadian Customs at the port of entry, this being YUL ,regardless of a "GVA YYZ with a stop at YUL" or GVA YUL YYZ" itinerary. You will have to deplane with all your stuff and clear customs and come back around again.

Another thing to consider is the slightly different AQMs you will/might get. Your GVA YYZ "YUL option" will be calculated as an international flight despite the stop and onward domestic... the "non-YUL" option will calculate GVA YUL as an international trip, but the YUL YYZ as domestic... it's all about whether you have one flight number or two flight numbers.