Currently, in my neighbors there are at least three users I already follow for a long time. Plus, when they show up in my neighbors, it displays the red circle with the + sign, as if I don't follow them.
Hi, the first one is not really a bug, strictly speaking -- there's no reason why someone you're already following might not be considered a neighbour. However, I can see why you might want to only see people you're not currently following.
The second one, not indicating the neighbours you're already following, is definitely broken, and a bug ticket has been written to address this problem.