Just copied from my local Nextdoor.
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"Supermarket scam!
Mum has just come back from shopping.
This has happened outside Taplow Tesco recently.
People appearing to be store workers in similar uniform offering 20% discount in store today if you swipe your card on their hand held device.
An elderly gentleman has had his card swiped and bank account cleared while he walks around the store. Because he’s given his card he won’t get it back as fraud.
While it’s getting busier out there, please be extra vigilant. X"