Although it seems the OP is doing it for the purpose of mileage earning, another legitimate reason is in order to protect seats. I was booking award tickets for a friend who already had a round trip GUM-ROR-GUM, but then realized he needed to go to SFO for a few days immediately afterwards. The only available routes from ROR went via GUM, so the cheapest way without backtracking was to get a one way ticket GUM-ROR, and an open jaw ROR-(GUM)-SFO-GUM. In order to be sure not to loose any seats I wanted to book the ROR-SFO-GUM itinerary before changing the GUM-ROR-GUM ticket to a one way, leaving me with a double booked ROR-GUM segment for a while. I explained the situation to the agent who advised against it, so I took the risk and changed the ROR-GUM leg right before booking the ROR-SFO-GUM.

I am not sure if the cancellation is immediate, especially if you wait putting in the FF#, but in my case the agent advised against having the same person booked on the same flight even for a few minutes.