- Adding the same instance of a teardown more than once is the same as adding it once
- Removing a teardown with `Subscription.prototype.remove` will remove it much faster

BREAKING CHANGE: Adding the same function instance to a subscription as a teardown multiple times will now result in that function being executed only once on teardown. This brings us inline with the behavior of EventTarget, and also makes removing teardowns faster. The workaround is to make sure you are adding a new function instance to the Subscription each time if you need the same effect.

Resolves ReactiveX#6400