Sessions in Laravel 5.0 keep piling up

I have set sessions to be stored in the database. just one user is logged in and after a couple of minutes 700+ sessions are stored. the system uses a lot of ajax calls, but that shouldn't be causing this.

This is how session.php looks like:

return [
    'driver' => env('SESSION_DRIVER', 'database'),
    'lifetime' => 1200,
    'expire_on_close' => true,
    'encrypt' => false,
    'files' => storage_path().'/framework/sessions',
    'connection' => null,
    'table' => 'sessions',
    'lottery' => [2, 100],
    'cookie' => 'laravel_session',
    'path' => '/',
    'domain' => null,
    'secure' => false,
];

Anybody have a clue?

via Chebli Mohamed