When 1000 miles is offered, it is posted as 500 each way. You won't gain anything only using one way of a RT ticket because usually one way fares are more expensive. I have booked a cheapy RT and only used one direction (ORD-LGA) and it was a lot cheaper than a 1 way would have been. I got 500 miles, not 1000 because I didn't fly the return.

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DtG