I am flying to JFK and to Newark, aren't I theoretically competing against myself for doing that?? Should I remove one of the flights??
No, you are not competing yourself. Every airport, no matter if attached to the same city, has it´s own demand.
So you can offer flights to Newark,La Guardia and JFK at the same time without loosing any PAX.
It will be (somewhat) the case for the planned city-based demand system.But that´s still beyond horizon.