Just cancel the return on the free ticket, and you'll be all set. And even if you did leave it, I doubt it would create a problem ... at least with Easy Chicken. It would simply pop up your name twice, and ask you to select the correct itinerary. If you use an agent, the agent will punch in your flight number and name, so they probably won't even see the other record. It would be a nice gesture to cancel the seat you don't plan to use so that someone else can have it, though.