I always say, "it's so nice to see you". Leave off the "meet" or "again". Chances are, there will be something in next conversation to help you remember.
If you get stuck in a "we've already met", say, something like "I thought so, but I met so many people today/this week that I wasn't sure!"
Don't say that you have a bad memory or are bad with faces. It's rather innocuous, but it's best to not put yourself down in recruitment.
Good luck next time!
__________________
|