assertUnique might break for collections that are not integer-indexable

### Description

Since we use slice indexing, I think `assertUnique` will fail on collections which are not integer-indexable. Sets have this property, but sets already enforce uniqueness so that's not a real problem. [multisets](https://pypi.org/project/multiset/) may expose a problem here though?

Start by locating the assertUnique entry point and reviewing how it uses slice indexing. Reproduce the reported behavior with a non-integer-indexable collection such as the linked multisets package, then add a regression test if the failure is confirmed and verify the existing uniqueness checks still pass.

Written by the indexing model from the issue text.