Just remember, you're entitled to your line through a corner.
A good competitor will see you approaching the corner and realize they don't have the room to make a pass before you begin your entry. They should just tuck in behind you and follow you through (they actually get BETTER times following the best line than if they stick themselves where they don't belong and cause contact).
A lot of what online races lack in GT5 is foresight. In turn 1 lap 1 no one is going to get the best line, everyone must be curteous to those around them. But one on one racing requires the kind of planning I mention above. Keeping tucked behind until there actually IS a place to pass. That means when you hit turn in, the other driver is ALREADY side-by-side and taking the inside line, if it's a well done pass.
This is much different than poor planning, braking late and assuming the other driver will give up their line so there's no collision, which is bad racing form.