Hello,
When you create a serial manually in the backend, there is no check on duplicates.
These checks are just made during imports (and you can deactivate it in the backend).
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.