Taxes - Do I have to pay twice?

Hey all,
Question for you regulars please
I want to book a miles flight. I was planning on booking as soon as the dates become available for the outward leg, and then booking the return leg when it too becomes available. Two one way bookings. I just tried a 'test run' and it shows that I have to pay 1k taxes LGW-MCO leg and then if I do a one way back I get charges 1K USD taxes and yet, if I book it as a return, I only have to pay the 1k once.
Why is this please? And is it right?
Question for you regulars please

Why is this please? And is it right?