A1: Disabling the option would indeed prevent you from getting multiple downloads.
A2: Both repack and proper are considered equal upgrades by sonarr. There’s an open issue somewhere that discusses the merits of considering them the same/different.
Sometimes I also kind of wished that sonarr would ignore repacks, because it’s usually crap like “lol forgot the nfo/proof/… so let’s give you the entire freaking set of files again with one minor change”.
A proper indicates there’s a serious problem with the original release. “Official” rules say you should get rid of the original release and get the proper instead.
On the other hand there are plenty of release groups that fuck up a release (e.g. the video stops playing halfway through) and then release a repack, while they should have proper-ed their own release. My opinion.
So it’s still worth it to get the repack (or check out manually what’s “repacked” about it and decide if you should get it or not).
About the option in sonarr: I feel it’s a good tradeoff between getting the same file again once in a blue moon, or in most of the cases get a fix for a broken file.
So I keep it enabled and let sonarr handle both repacks and propers automatically. For me it’s not worth the “risk” of discovering I have a broken file, sometimes months after it was released and I have to find a working one manually.