Yeah, I would expect it to be hard, similar to asking an llm to substitiute all letters e with an a. Which I'm sure they struggle with but manage to perform it too.

In this context though it's a bit misleading explaining the observed behavior of op with that though, since it implies it is due to that fundamental nature of llms when in practice all models I have tested fundamentally had the ability.

It does seem that llms simply don't use double spaces (or I have not noticed them doing it anywhere yet), but if you trained or just systemprompted them differently they could easily start to. So it isn't a very stable method for non-ai identification.

Edit: And of course you'd have to make sure the interfaces also don't strip double spaces, as was guessed elsewhere. I have not checked other interfaces but would not be surprised either way whether they did or did not. This too thought can't be overly hard to fix with a few select character conversions even in the worst cases. And clearly at least my interface already managed to do it just fine.