同--together, 乐--to be happy; happiness, 同乐--to be happy together. 同乐 is a courtesy of reply when someone congratulates on you. 同乐同乐--Saying it twice is to emphasize the feeling. It's usually used on festival when a host says to a guest. 例如： A: 新年快乐! B: 同乐同乐! The other use of ”同乐“ is ”与民同乐“, a longer story, but it's not relative here. If you want to know, message me please!

it's party time :P ok, maybe not that exaggerated :P

It's like "Same to you" when you say "Happy ```whatever day" to others.

It can be understood like"u too,have fun".

well,乐may be explained as happiness in english, it means someone wish you the same happiness as themselves!It's a common way to speak when you are congratulated on any occasions!