Nope! If you've selected seats, or paid for a carry-on bag for your trip, your purchase applies to all legs on one direction of your journey. If you've booked a roundtrip, you'll need to purchase Extras for both the outbound and returning, which you can do either at the time of initial booking or through Manage My Booking after you've purchased your flight. If you've purchased and selected a seat, that will be your seat for all legs of your trip.