MyBB SQL Error

MyBB has experienced an internal SQL error and cannot continue.

SQL Error:
1062 - Duplicate entry '0' for key 'uid'
Query:
REPLACE INTO mybb_sessions SET `uid`='0',`sid`='21db20ab1e253e833f5cff4fc341a52b',`time`='1505911884',`ip`='54.198.247.44',`location`='/forum/showthread.php?tid=33',`useragent`='CCBot/2.0 (http://commoncrawl.org/faq/)',`location1`='61',`location2`='33',`nopermission`='0'