If I am flying from CDG -> YUL -> YYZ where will I clear customs?

The way I understand it now I collect my bags in YUL, clear customs, then recheck my bag in YUL to YYZ. If this is the case, how would carry on duty free work? Do I have to clear security again in YUL? That means I wouldn't be able to carry it on the flight onto YYZ.

Any insight is appreciated.