We have been using the same online registration system with CiviEvent and same payment processor for some time. Suddenly, registrants are getting an error message after submitting a payment through our online registration. The person is registered in CiviCrm and the payment goes through, but they still get the following error message:
Sorry but we are unable to provide this at the moment. Could not find valid value for id. Return to homepage.
So what happens is that after they pay, they don't know if it went through and then they try to register again so we are getting people who double register.