Forgive my insipid prose but for clarity's sake I'll describe step by step what happens for me.
I add product into cart. I view cart. I click "proceed to checkout." I enter my information (including all required info) and click "proceed to confirmation." Confirmation page loads, I click "proceed to Checkout" and up pops the previous details page again.
I've tried this in test mode, in live mode, with paypal sandbox emails/info and real-life emails.
There's probably a simple fix to this and I'll kick myself when I find it. At least I hope so.
If you need anymore info to diagnose, just ask. Thanks in advance!