I think it's technically legal for you to even cancel the return leg if you know you aren't going to use it, rather than no-show. Throwaway ticketing is prohibited, but they can't make you fly a leg that you booked sincerely intending to take it and then your plans changed. You would have to do this a lot of times before they would give you any hassle over it.
On duplicate bookings in general, I've occasionally even had two seats in my own name with the same MP number on the same flight and they didn't notice.