That's awesome. When you hold the button down long enough for it to repeat, you're actually pasting many copies at the same location, which you can't see because you have 'offset overlapping media items vertically' disabled. I'm not sure if the behavior you are seeing where it sometimes adds a track is related to some kind of overflow when you have a thousand media items at the same location, but I can't immediately reproduce it. In any case I think holding the paste button down is probably not something you want to do even though it looks cool