Using your numbered guest list, write the corresponding numbers in a light pencil on the back corner of each RSVP card. It will lessen any confusion if a guest sends the card back and you can't decifer their writing. I have also had guests forget to write in their name at all!
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My friend commented that this was also very helpful at her wedding. Being a Korean couple, there were 25 LEEs and 30 WONGs in their guest list. Not easy to figure out if someone replies "Mr. & Mrs. Lee", unless you number your cards.