Hi @tango13,
It sounds like the review_form_responses_unique index already exists in your 3.3.0-x database, but that index was only added to OJS with the 3.4.0 release.
Did you already try an upgrade which failed, then re-try it again without reloading form backup? That’s most likely the cause of this message. Make sure to roll back to a clean 3.3.0 backup before running an upgrade again.
Regards,
Alec Smecher
Public Knowledge Project Team