⭐ Merited by WEB503 (1)
There is no rule for a duplicate translation 
The only rule stating anything about duplicate posts states, in fact, that translations are permitted:
The only rule stating anything about duplicate posts states, in fact, that translations are permitted:
12. No duplicate posting in multiple boards (except for re-posting topics in the local language boards if they're translated [...]
From that rule I can draw a conclusion, that the translation is permitted indeed, however, it can be assumed that we are talking here about the first ever translation, not the next versions of that translation into the language, into which some translation was previously made. That is what internetional meant, as far as I can see. And in this case it would be better to modify the OP in the translation that already exists, rather that create another one. Or simply add updates in the translation thread making a new post.