I don't know Japanese and I cannot fully answer this myself.
I can say this, however: It's quite common that messages have the same content in English as other messages, and this is OK. This is useful because quite often they have to be translated differently to other languages, although sometimes they are the same is other languages.
It's up to you and other people who know Japanese and use Wikipedia and other MediaWiki site in Japanese to decide whether they should be the same or different in Japanese. If you are not sure yourself, you should consult with other people who know Japanese. If you decide that they should be the same, just change them.