What Are The Repercussions Of Taking An MMR Booster Twice?

We're traveling to California with my 4-and-a-half yr old and 2yr old. I don't know if my 4-and-a-half yr old has had her MMR booster yet (Her card hasn't been updated), Is it harmful if she has had it already, and she gets the booster again (tomorrow)?
Also, my 2yr old received the vaccine once when he was 1. Is it harmful for him to receive the booster tomorrow since he's only 2-and a half?