DefUseChain can output the same reference multiple times with nested loops

[LonelyCat124]

There is a bug in DefUseChains with nested loops where the same loop can occur multiple times.

Helpfully the comment in the code says:

                        # We will only ever reach a reference once, so
                        # we don't need to check uniqueness.

at line 367, which seems to be incorrect, so we just need a uniqueness check here.