When you make a mistake at work, do you tell the person in charge, or hope they don't find out?

I recently gave a woman a duplicate £30 item, not knowing she already had it in a bag along with some other items, a staff member informed me and told me they would keep it quiet but I still told my manager because I've always believed that honesty is the best policy. Turned out she didn't really mind anyway.

Tell themKeep quiet