This bug has been fixed https://en.numista.com/forum/topic162476.html
Now when I duplicate a page such as https://en.numista.com/catalogue/set.php?id=327 (unpublished) with 6 coins X 2 mints, only 6 coins X 1 mint are kept https://en.numista.com/catalogue/set.php?id=328 (unpublished). All 12 coins/mints have to be re-entered (in order to keep the sorting correct as discussed here https://en.numista.com/forum/topic162320.html).