How can I say "Congratulations again" in Japanese?
Is this sentence correct? "改めておめでとうございます"
please tell me if there is a better one :)

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That sentence works perfectly. If you want to be even more formal you can say "再びおめでとうを申し上げます。”

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thanks :)

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"改めておめでとうございます" is perfect.

But We don't say 
"再びおめでとうを申し上げます。”
and usually say 
"改めておめでとうを申し上げます。”　
　 or　
改めてお祝い申し上げます。

お祝い = Oiwai (Celebrations)