Hi,
If you want to ask for a field during the checkout ; the best would be to use an "order custom field".
To check if the entered value is unique, the usage of a custom plugin would be required.
Thanks to the plugin and the usage of the trigger "onBeforeOrderCreate", you can authorize or refuse the creation of the order.
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.