I need help with this as im not 100% on what i need

I have changed from a 3" blitz nur to a 2.5" afterburner decat centre pipe and racebox backbox. My question is when i removed the old centre pipe it only had a standard gasket but the new centre pipe has got the lip on it for a doughnut gasket. if i use the doughnut do i still need to use a flat gasket between the join as well

any input welcomed

thanks lads