Depends, really.
First swipe (on little keypad jobbie) is to capture the details for the credit card handler - so the transaction can be authorised.
Second (on the till) is to capture the details for the shopkeepers records in case of query down the line.
It used to be reasonably common, but I don't see it much any more because most of the POS systems are sufficiently integrated, or else the retailers don't worry about keeping their own independent copy.
In the good old days they would have kept a copy of the details by keeping one of the paper slips.
z