CreLoaded PayPal checkout Problems

This job is for CreLoader Expert only, my website has been modify with lots of add on features, now the PayPal checkout has lots of problems, most of times, it cache pervious customer's info, and when check out, the PayPal site alway give me error saying the payment has been paid! Return to Merchant This invoice has already been paid. For more information, please contact the merchant. If you are a CreLoaded expert, please help me to fix the bug(s) ASAP. :) Thanks,