Spring change to deque implementation

Comment From: wind57

I do not know all the complete internals, but I found this while working with a different spring-project. Specifically, here. Accidentally, I looked up the implementation and I though this can be improved.

And while that code has it's problems (that I will issue a PR for), we can start fixing from here, imo.

Comment From: wind57

I missed the fact that these are supposed to be unique, a Deque is no good.