I don't think it's even possible to spell an onomatopoetic word wrong. "Bow wow", "woof woof", "arf arf", etc. need to all be accepted. I'm not learning Finnish when Duolingo tells me that "bow wow" is not an acceptable way to say "woof woof".
Here's a thought: Maybe it's best not to use such words at all in a language course, since spelling, usage, etc. are all so variable even within a single language, let alone across several.