Providing context always helps get a better answer.
"You can say that again!" is an expression of agreement with what someone else just said. Meaning "I liked hearing what you said and would like to hear it again." It is not a literal request for someone to actually repeat themselves.
One co-worker to another: "It's finally Friday and we don't have to come to work tomorrow!"
Other co-worker: "You can say that again!"
Student or Learner