I made this cartoon in 2000 and since then I've seen several comics that used a very similar premise.
It's not that I believe any "thievery" was involved but a joke like this is basically low hanging fruit on the comedy tree - it takes only a tiny jump in your brain to arrive at this kind of simple joke.
None-the-less, I was happy to have been the first one to arrive at this joke until recently when I was reading a book on comedy and they listed as example a joke from a comedian in the 1960s who did a bit based on the step-ladder premise.
Well... damn. I guess it has been done before. Now, I'll have to find something else to feel smug about ;)