If you are getting the duplicate transaction or invoice message it is because you are sending duplicate values to PayPal for a transaction that has already taken place. This is a security technique used by PayPal.
Getting this error indicates you are sending the same values more than once. If there was not a successful transaction the first time you would not get this message. You say:
I tried opening another browser and nothing works. This only happens on my second transaction.
On the first transaction do you get to the checkout success page? If so what did the page tell you about the session, did you see any of the debug statements? If you did not see any please post back with your checkout success page.
In regards to your statement that you tried another browser and nothing works what other browser did you try and what is it that is not working? Please be specific and provide as much detail as you can so we can help you get this resolved.
This is not a complex problem and should not require a phone conversation to debug. If we can run some test transactions and get back info about the session this is what will help solve the problem.