I mean there are only so many notes, the combination must have been all covered by now, right? So some overlap between songs is to be expected I assume.
I think that depends a lot on what your definition of "close" is. If you have notes and lengths, you can generate a fairly big space just from that. I'm no music expert but you could ask whether transposing the notes up and down the scale counts as a new "thing", along with any number of other ways of generating a new tune from an existing one.
