And that's why...
I saved a seller on a popular auction website from potentially receiving negative feedback in the near future. I informed that seller they listed a duplicate auction for a serial numbered trading card.
If I didn't intervene, they could have potentially lost their 100 percent positive feedback, because one buyer would have gotten their order canceled. That's catastrophic in the auction market.
If a shady buyer discovered that duplicate entry, they could have extorted the seller into offering a better trading card in exchange for positive feedback.
And that's why you should always examine your listings whenever you add a new listing...