Itu adalah sebuah adat istiadat yang unik dari Jepang.

English Translation

That is a custom proper to Japan.

Tip: KBBI says “istiadat” is a synonym of “adat”, but “adat istiadat” is a proper set phrase.

code of conduct that is eternal and passed down from generation to generation as an inheritance so that it is strongly integrated with the behavior pattern of the community (translated by Google Translate)

According to the University of Leipzig Indonesian Corpus (LCC), “adat” ranks the first for the collocation of “istiadat”. So, it’s not a redundant unnatural phrase at all.