In Emacs, undo makes an undo pointer go down in the undo stack. Pressing undo again goes back another step. If you do any other regular edit, the pointer starts over at the top of the undo stack. Undo puts its own edits on top of the stack like any other command.
So if you "undo, undo" you undo two things. If you "undo, edit, undo", you're keeping the first undo but reverse the edit. If you "undo, edit, undo, undo", you're back to where you started (except your undo stack has now grown).
So if you "undo, undo" you undo two things. If you "undo, edit, undo", you're keeping the first undo but reverse the edit. If you "undo, edit, undo, undo", you're back to where you started (except your undo stack has now grown).