Which Twitter Unfollow Tool can achieve this?

I have tried a few tools for unfollowing people who do not follow me. However I have not come across any tool which will prevent following the users whom I had unfollowed before. I do not want to follow and unfollow the same users again and again.

Is there any tool or app which has this feature ?

With Twitter dub you can set users to a whitelist so you won’t unfollow them