Creating another instance of a feature in Font Info > Features does some bad stuff.
Whatever you do in one instance of the feature gets passed to the other. This usually means that a new, blank feature will overwrite its non-blank namesake, even if the original is set to auto generate, and is usually protected.
I can think of situations where it would be useful to have a hand crafted set of feature rules alongside the auto generated ones, so being able to create two instances of the same feature would be the way to do this.
If that’s actually a bad idea, Glyphs should not allow the creation of duplicate feature names, and should throw an error if the user tries to do so.