I'm being told by Paypal support that my payment flow does not support the unique id feature and the only way we can get our transactions to be completed is to configure our dealers account is to allow duplicate invoices. Our concern is if we allow duplicate invoices then we run the risk of our customers being charged twice. I was told by support that there is an enhancement request that, if implemented, would change this functionality; however, this is not a bug. Anybody have any experience with this and can recommend a solution?
Its better to use the COMMENT1 and COMMENT2 variables to store extra information that you need for your database. It is true that if you are putting this information in the invoice number, and you have duplicate suppression enabled, you will have error messages. If you disable duplicate suppression, you do run the risk of your server sending two api calls for the same payment, causing your customers to be charged more than once.
This is why I am recommending using the COMMENT1 and COMMENT2 variables.