Paul said it twice for emphasis, “Rejoice in the Lord always, again I say rejoice.” Really he said it three times, for back in Philippians 3:1 “Finally, my brothers, rejoice in the Lord!” It is no trouble for me to write the same things to you again, and it is a safeguard for you.” Repetition is a way of saying, “This is really important.” There were not many things that Paul repeated as he did this. He knew that if they enjoyed God, much of the rest would fall into place.
By the way, note the form or this verse. It is an imperative, a command. This is not in the “nice to have” section of Christianity. If we are not enjoying him, we are sinning violently against him. We are disobedient. There is no middle ground.
Josh Hunt. People Who Enjoy Their God.