I think it's called "back-to-back" ticketing.

Consider this: I usually get a RT booked by the company, say BUF-BHM. Out on a monday, back on a Friday two weeks later. For the weekend in between, I can get a BHM-BUF RT. No problem, all in my name, my FF number, etc.

Of course, I actually take all the trips so there's no problem.

I don't know how they can catch you, and this is ethically a grey area, though most airlines try to discourage it. Not sure why: they get paid and wind up with an extra seat they can sell that's already paid for. And it can't be any sort of technical reason - WN will sell you one-ways all day long.

I thonk this is a holdover from the old days, and we (the airline) do it because ... because ... because we;ve always done it, so there!