Somehow you've already got an entry in the table with that exact same key - so you will need to use PHPMYADMIN to try and repair the database corruption.
Please note that whilst rokgallery is a free extension, support for it is not free and requires that you have an active subcription. Our priority will always be to members with active subscriptions.
Regards, Mark.
Please search forums before posting. Please make sure your post includes the version of the CMS you are using and a link to the problem. Annotations on screenshots can also be helpful to explain problems/goals. Please use the "secure" tab for confidential information.