Duplicate database entries

Check the database for a duplicate entry, if an entry exists return.

The patch looks good, but seems it can't resolve the problem ulitmately.
The main reason is because$query->condition('s.timestamp', \Drupal::time()->getRequestTime()) may not the same on page redirect.
So the patch can reduce the problems, but can't resolved it ultimately.

commited, add condition with uid check.

Automatically closed - issue fixed for 2 weeks with no activity.