First of all, you will almost never have one of those in a perfect condition. Chances to find one in a good condition (like brand new) is like the chances of wining the lottery ticket. Its almost impossible.
Second, I dont agree there might be a lot of these. Whats not reported is technically not existent until it is reported. How many collectors on this forum have one at hand right now? 5, 10, 20? How many were declared to be found? 30, 40, 50?
Third, the release of polymer banknotes will have effect on everything. These polymer banknotes will eventually replace every single cotton made banknote in Canada. In other words, you would never see a cotton banknote again after a while. So if you have one like the double denomination one, it would always be special regardless of time.
Polymer notes might also have their mistakes, but who knows if you will ever have double denomination errors on them. Google double denomination polymer notes and check for yourself.