As the cow pointed out, booking the same O/D pairing on the same date is against the tariff. However, I would disagree that it's "going" to get picked up by the system, because the dupe detector is incredibly stupid. I would say it's extremely likely that it will be picked up by the system, but there's a chance it will slip through.

You should also beware that although the system will generate a note on the PNR that says something like "clear duplicate by 7AM Montreal time on August 28th or AC will cancel" (a concierge or res agent once told me what it says, can't remember the exact wording), you will most likely not be notified of this flag. One day you'll simply log in to check on your bookings, and like Keyser Soze, it's gone. Then you'll have to call AC to get it refunded, and they'll only refund it in accordance with the rules of the fare, which may cost you fees. They may cancel the one you were more likely to want to fly, or it may be the other one. They may cancel the one you booked first. Or they may cancel the one you booked second. There's no way to predict it. And they likely won't reinstate the cancelled itinerary for you; if you're lucky, you'll be able to re-book the same thing for the same price, but of course there's no guarantee.

As for booking on back-to-back days, it's incorrect that the system will cancel what's impossible. It will cancel what it deems illogical. But remember, it's very stupid. It will sometimes not cancel things that are impossible. But it will also cancel things that are very possible to fly, with legal connections. For instance, no one could possibly want to fly YYC-YVR-YCD-YYZ-YYC on the same day, or YYC-YUL-YYC on back-to-back days. So it will cancel some portion of these things. Even though they're composed entirely of legal connections and do not violate the tariff in any way, shape, or form. But you may also book three different flights from YYC that will all be in the air at the same time, and it may not cancel those. Because it's stupid.

Again, with this very stupid tool (and yes, obviously it's the fault of the people who coded it, but I'm trying to be nice and not call them stupid), nothing is guaranteed. So I would say it's highly likely that your back-to-back days will be flagged and cancelled. It's not impossible for it to slide through, but it would be unwise to rely on that. Again, if it gets marked as a duplicate, you likely will get no warning.

If you go through this thread, you will find some people have suggestions for avoiding the dupe detector that have been successful for them at times, but I don't believe there's any guaranteed way around this, so would not book anything that would upset this very dumb system unless I was prepared to risk the consequences.