Wow that would be really bad. I mean that would get you on the top of the world and dreaming about the next year and then just destroy you.
Actually, in addition to CMU, Johns Hopkins, MIT, and UC San Diego have done that too (accidentally, of course).
Reference: https://www.usnews.com/news/articles/2015/02/18/carnegie-mellon-mistakenly-sends-acceptance-letters-to-800-students
I really hope that doesn’t happen to anyone in the future.