[Spivak]

I'm confused by your question, lists don't enforce uniqueness—you can do ["a", "a"] and it's perfectly valid. Sets and the key values of hash maps can't have repeating elements. You can for_each over lists that come from a data block so you can't statically know if a list is effectively a set.

If you don't want .0, .1 then you agree with Terraform's design because that's the only way you could for_each over an arbitrary list that might contain duplicate values. Terraform making you turn your lists into sets gets you what you want.