I recently wrote about how to determine who gets a "plus one" to the wedding. While there are some solid criteria to consider, you will likely find yourself in a situation where a guest has RSVPed for two but their invite was only for one. So what do you do?
(I guess this is a little premature for me since we haven't sent out our invites yet, but I think this is what I would do.)
In my opinion, when a guest RSVPs for two when it should only be one I think it's best and easiest to just pick up the phone and be honest. I know that can be awkward and you don't want to make them feel bad, but it has to be done. And imagine how the person would feel if they found out after the fact! Remember, this is supposed to be your day and, maybe even more importantly, it is your dime and an uninvited plus one means extra dollars out of your pocket.
So here is how the conversation should go:
"Hi _________! We're so glad you can make it to the wedding to celebrate with us! I noticed your RSVP was for two but unfortunately we only accounted for one in our planning. I hope you understand! It would be great if after the wedding we can all get together and grab dinner. Then we'll have fun honeymoon stories to share."
See, that's not awkward at all and the other person will understand. Just make sure you make them feel welcome and encourage a get-together after the wedding. Good luck!