Manually following process URL results in a crash

[Goury]

Flow:

1. Pay for something
2. Be redirected to schema://domain/urlspace/process/some-long-uuid-string/?paymentId=PAYID-CAPSANDALSONUMBERS1111&token=EC-SOMEMORECAPS&PayerID=AFEWMORECAPS
3. Be redirected to the successful view
4. Manually visit URL from 2 for no particular reason
5. Crash

Expectation:
Not crash but just a 400, because the server had done nothing wrong and just visiting a link one found in ones browser console shouldn't flood admin email with crash reports.

Crash happens at payments/paypal/__init__.py, line 270, in execute_payment

Please fix

[Goury]

Also FTFY

Not sure if my solution is correct or good, but it works in my case

[WhyNotHugo]

The provider code should check that the payment is not yet paid before calling execute_payment.