There may be other ways to handle this--I make no claims this is the best. This is just what I'd prefer to encounter if personally asked to unconsider a node.

Notify an editor, if one's around (or use the chatterbox), and wait with your consideration vote to see if the editor needs it to unconsider--unconsideration needs a certain number of keep votes or a certain number of edit votes, and the post might be lacking one of the right kind of vote to unconsider it. If you get no response to your message, and the problem hasn't been dealt with in about 5 minutes, the editor probably isn't there.

If no editor is around, vote keep and message the editors as a group. With only two keep votes deletion won't happen. If you explain the reason in your message ("221876 was thought to be a duplicate of 221901, but in fact it isn't") the editor will spend less time verifying and be able to unconsider more quickly.