Some posts just keep growing and growing and then get cut and then grow some more. Sorry. Read it or ignore it as you wish.
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This REALLY needs a video if the OP has the replay saved, there's not much else we can say. But it sounds like the OP ran to the outer line when he should have been more cautious.
Just a note, we already know the OP wasn't looking out for him, he said in his description!!! So how does the OP apparently now know that the follower's 'bumper was in-line' with the rear of the OP's car, he could have been anywhere!!!
I think this mostly sums up what has kicked up the amusing discussion in this thread. The OP was not very clear initially about exactly what happened, and has clarified things a few times without managing to make it much more clear. So a lot of people have been reacting to and in some cases arguing about an incident which exists only in all of our various imaginations.
I was all set to jump to conclusions this morning about what happened in my imagination before realizing that that wasn't the actual incident. Without knowing the exact specifics of who was where and when it's tough to make any real determination. It could well be that the overtaking car was never alongside by more than a couple of inches and had no business staying there. It could also be that the overtaking car was substantially alongside going through the corner and thought it was still a side-by-side fight(even if a losing one) and the defender decided he somehow owned the road and ran the overtaker off. It could also be that the overtaker was ready to abandon the move but saw what appeared to be a substantial mistake and decided to try to capitalize on it.
A lot of the discussion probably was kicked off by the statement that the OP didn't see the other car out of his windshield/forward view, and apparently doesn't feel that other drivers deserve the respect of using [GT5's unreliable] mirrors or using the buttons to look sideways/rearward or having the minimap enabled so as to help avoid running into them. The statement that the OP owned the racing line also suggests a certain attitude. The OP has since claimed that they hit rear bumper to front bumper but since he apparently wasn't aware of where the overtaker was it's tough to make that claim.
To actually give an opinion on the theory of all this:
I do tend to believe that in most cases it is always your responsibility to be aware of where a car you are racing with is. Just because you don't expect someone to be there doesn't mean you have the right to not pay any attention. People don't have the right to try to make last second moves resulting in only a tiny overlap(the "one wheel is not enough" thing) and expect the lead car to know about it and sacrifice tons of speed in a last second attempt at avoiding contact. In those cases the following car has a much better view of where each car is and so the burden of avoidance falls with them. Even with adequate visiblity and alertness, it's very difficult to know that someone has just pulled six inches alongside you. But if there IS a fight with some proper side-by-side racing, the racing isn't over just because one driver thinks he's won it, even if he does have the advantage(even a substantial advantage) for the moment. I think this is generally what I've seen in real-world racing, though the specific rules vary.
You also have to assume that the other driver in an informal online race is not going to exercise the same discretion as a driver who's livelihood and even life depends on knowing when to give up on a battle. If they're alongside you, they're alongside you until they aren't -- not until you think they should give up. If you go into a corner side-by-side, you should expect to stay on a side-by-side line all the way through the corner or until you have actually pulled fully in front or fully behind. The contest is not decided if one car is a bit ahead at the apex. You can make things difficult for them, but you still must leave them enough room to stay on the road.
The overtaking driver probably overreacted in this incident, whether "racing incident" or fault one way or the other. But racing drivers do tend to be rather emotional after incidents. Gamers are not exactly renowned for their extreme levels of maturity and calm either.