Identical contacts created when resubmitted a event sign up from by going back from offsite payment page

Scenario:
1. An offsite payment processor is used on a event registration page;
2. An user filled out the event registration page and click on submit;
3. It takes the user to the 3rd party payment page;
4. User decides to go back to the event registration page to review again;
5. User click on submit button again;

After these steps, two identical contacts are created.

This might apply on other payment pages as well.