In the UK that only applies to unsolicited goods, not to genuine errors. So, if Nintendo were to send you a switch and then ask you to pay for it, sure, you can ignore the demand. If you buy a switch from Nintendo and they send you two you have no right to keep the second (or third, in this case).

That said, if the seller makes a mistake then they need to rectify it entirely, they can't demand you wait around for a courier at an inconvenient time or get you to pay for postage.