How is the solution supposed to work, exactly?

But that sequence isn't allowed, because one of the assumptions we've made is that if we're working with an application, and if a single value repeats, the whole sequence does. In your example, we have ,2,3, and ,2,1, which contradicts this.

I think it would be best to clarify if an unwinnable state counts as a draw.

This might just have been the best Kata I have tried so far. It was, at least, the first one that challenged me past the naïve approach, and I even learned things I wouldn't have otherwise. Thank you for making this.