When our daughters were 1 and 3, our youngest called the older one, Who-Who instead of Crystal. The oldest one called the younger one Ra-rach instead of Rachael. We could understand Ra-rach, so much that it almost became a nickname, before we nipped it in the bud. But, we could not figure out Who-Who.
Years later, when they were teenagers and we lived in a two-story house, I stood on the first floor and called up to Crystal, “Crystal, Woohoo” (or however you spell that call we make) and then stopped in pause. Well, I guess that is where Who-Who had come from!