Bug where proxies can get duplicated even when `exclude_used=True`

[CosmicHorrorDev]

So adding the test in 97e22b9 has revealed that duplicate proxies can still get out because the check for duplicates is done during ._fetch() while the blacklist isn't updated till .get_proxy() or .get_proxies(...). This can cause duplicates to get through that are added to the internal proxy list before the blacklist is updated.

[CosmicHorrorDev]

This has been addressed in ef361e2. Glad that adding tests has already started paying off.