Hi,
There is no options by default in HikaShop for that, but this can be done by creating a new plugin, so PHP knowledges required.
The plugin can be based on the function "onBeforeCartUpdate()", in this function do the MySQL request to check that the value of the item field is not already in the database, if already in so set the "$do" to false, else let it to true.
Here is the developer documentation:
www.hikashop.com/support/documentation/6...umentation.html#cart
If you are not a developer, you can still post your request in the commercial jobs section of this forum.