I just recently crossed the 3k line on Stack Overflow, and given the huge size of the review queue I immediately started reviewing.
However, before having the necessary privileges to cast close votes I flagged questions for closure. One of these questions now popped up in my review queue, but when I clicked Close the dialog simply stated that I already cast a close flag on that post, and left no option to kind of convert my flag into an actual close vote.
I would suggest to remove those posts from the review queue - as I did not recognize the question immediately and spent another two or three minutes on reading the question and its comments again...