The solution two this error is one of two things, either session create is not checking for an existing session before starting one, or the php.ini has session.auto_start enabled.
Make sure you create a session by checking if one doesn’t already exist:
if(!isset($_SESSION)){ session_start(); }
If you are sure this has been set properly then the following could be the solution:
// php.ini session.auto_start = 0
Restart apache / php-fpm and see whether this solves your problem