This happened to me a few months ago, too. I mailed the landlady a second check for the rent with the understanding that if the first check showed up, that she'd use it for the next month's rent.

I also notified my bank in case someone showed up trying to cash it, but didn't put a stop payment on it because it costs over $20 to do that these days and I just didn't want to pay it, although that may come back to bite me if it was taken for some nefarious purpose.

Roxy