You've got it right! Regardless of how many units are in a Montréal -plex, they should be tagged as `building=apartments`. The tag for what StreetComplete calls a "duplex" is only for homes that share a single wall with one another: https://wiki.openstreetmap.org/wiki/Tag:building%3Dsemidetached_house

The `building=*` tags can be super confusing... 😅