Yes, there's the possibility that two or more cards share the last four digits, but it's highly unlikely. The oods are 2 cards every 10'000. Normally on receipts only the last four digits of a card are shown for safety reasons (this is an anti-fraud measure known as PAN truncation, where PAN is the card number), so that you know which card have you used, but should you lose the receipt noone can know your full card number.