How do I prevent customers submitting duplicate PayPal Payments Standard orders?

Due to the nature of how PayPal Payments Standard works, it is possible for customers to submit an order more than once through PayPal against the same shopping cart in SOLO Server. When this occurs, SOLO Server will decline the second order submitted because the shopping cart will not be found since it was deleted after the processing of the first order submitted.

In order to prevent this, SOLO Server passes the Cart ID to PayPal. PayPal then uses this as a unique identifier for a given order, and will only allow processing against the given value once, blocking all subsequent attempts. This functionality must be enabled on your PayPal account. In order to enable this functionality, follow the instructions at the top of the How do I avoid duplicate transactions PayPal support article to set the "Yes, block multiple payments per invoice ID" setting.