Buyers' scam of reversing their payment is awful; and there is no excuse for PayPal not fixing this loophole. It'd be fairly simple to do.
Duplicated payments, however, are not apparently reversible!
I'd had too many windows open that night, and forgot to refresh an earlier-opened page when I returned to it to make a payment by PayPal. I had been back to My-eBay, etc. and already made the payment (amongst others). PayPal's response to me, when I told them that I had accidentally paid one seller twice, was "send a message to the seller, and ask them to refund the 'extra' payment". I tried this, to no avail, and reported back to PayPal. There was apparently nothing they could do!
I believe it is still possible to double-pay for an item: PayPal have ignored my suggestion that a simple check be made automatically before each payment. Afore any payment is sent, it should be checked whether the item is still 'unpaid'.
Meanwhile, until PayPal add a milli-second check to avoid duplication of payments, buyers will have just one more thing to fret over....