Thank you for your kind replies, everyone! It's sorted!
I was only in the supermarket for a minute. Told me they'd already realized the mistake and had phoned HR to correct it. They were looking at me like I was really stupid. I knew that I wouldn't have been overpaid due to the clocking in/out system but I would have felt slightly dishonest and irresponsible for not telling them about the mistake. And I would have been obsessing over the thought that maybe there would some incredibly important reason that I should have told them.