"Fool me once, shame on you" means that people should learn from their mistakes. It's basically saying that it's not your fault if you're tricked by someone, but if you trust them and they trick you again, it's on you. You might hear it like this: [1]

“There's an old saying in Tennessee — I know it's in Texas, probably in Tennessee — that says, fool me once, shame on — shame on you. Fool me — you can't get fooled again.”