...my god

Set intersection is a fairly self-evident way to achieve a solution for uniqueness. The fact that the operator maintains the original order is a benefit.

I wouldn't label it cheating. It makes more sense to label it clever. I don't think labeling solutions as best practices makes much sense for this kata because it precludes the uniq method.

I'm wondering if this counts as cheating...

Aside from the intersection operation, you might be interested in the union, difference, cartesian product, etc interesting.

Redundant now, but yes, Set Intersection. Easy way to remember is that sets contain distinct items, and no duplicates.