In this case we would only just want to show User A one time especially since its a list of user activity. We can't store a hash and just push/pop because of race conditions (our first thought). If anyone has any ideas, I'd really appreciate it!
I don't follow :( Could you try explaining what the problem is in more detail, and how a binary tree setup would be better than a one time lock?
I don't really see the above example being a problem even if there is a race condition.
-Dormando
