The duplicate transaction error is trying to prevent duplicate transactions from happening. The last thing we want is to bill you twice. When this happens, the transaction has probably already gone through, but you didn't get the confirmation message for some reason.
Go to the home page by clicking the LexVid logo at the top left to check to see if your payment went through. Once on the home page, look at the Credit Hours Remaining section and look to see if it matches what you purchased (Bundle, A la Carte Credits, Unlimited Hours). If they match, you're good to go. You should also have received a receipt in your email.
If the transaction didn't go through, go to the packages page, select your product again, and fill out the payment form again. If that doesn't work, contact customer service at firstname.lastname@example.org or 877-327-1226.