How They Steal Your Bitcoins and Others

 When an attacker sends the same coin in rapid succession to two different addresses, the obvious outcome is that only one of them will get included.

Now, if you as a merchant don’t wait for confirmations of payment, then in a case like this, there’s a 50% chance you got the double-spent coin (and you won’t receive that money).
Your customer can trick you if they send the same coins again to their address.
Once the customer does both transactions, both transactions go to an unconfirmed pool of transactions. Whichever transaction gets verified first and gets six confirmations will be accepted, and the other will be discarded.

As a merchant, you might get the six confirmations first, but if the attacker gets the confirmations first, then you won’t receive your funds. That’s why it is said to wait for a minimum of six confirmations. 

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