A forum I'm admin on gets the following message when I visit:

phpBB : Critical Error

Error creating new session


SQL Error : 1062 Duplicate entry '89' for key 'session_user_id'

INSERT INTO r2ok_sessions (session_id, session_user_id, session_start, session_time, session_ip, session_page, session_logged_in, session_admin) VALUES ('29c757f07cdb88c484436314029a29e2', 89, 1520854824, 1520854824, '3306e4e0', 0, 1, 0)

Line : 187
File : sessions.php
The forum is http://r2ok.s4.bizhat.com/index.php?mforum=r2ok

Any advice on how to fix it? Thanks!