I think this is a circumstance where real life and GT5 need to be separated...one of the many haha.
If you are seeing this 'queuing' phenomenon in GT5, I can almost guarantee you that this is due to the poor AI. Whether the corner in question is a passing spot or not (regardless of how narrow it is), the AI will probably take it single file. There are very few corners in the game where an AI driven car of equal HP and/or PP will overtake an opponent on corner entry or mid corner (AI overtaking through means of late breaking or trying to hold inside position in a 2 wide scenario usually results in one or both cars ending up off the race track). The vast majority of AI overtaking occurs on corner exit (by simply out dragging the opponent down the next straight), or in the draft. To answer your original question, 'is it fair to overtake opponents that are taking a tight corner single file', I would say absolutely, go for it. If you are like me, and don't like to simply plow each AI car out of your way, then yes, you may have to 'queue up' in certain tight corners/sections (the hairpins on Cote d'Azure come to mind) to . However, that doesn't mean that you should feel guilty overtaking opponents under breaking going into said tight section.
As far as real life racing is concerned (by no means do I consider myself an expert, but I do think I have a decent understanding of most racing leagues) , there is no league, club, or professional series where a rule forcing drivers to take all, or just certain corners single file is implemented. There is always a chance something like this could be enforced in certain forms of amateur competition, but certainly not in a professional race. That being said, in certain forms of professional racing, it isn't unheard of to prohibit overtaking in a certain corner/section on a certain track. This would likely only happen in a race taking place on public roads, and would be specific to the track (sort of like individual ground rules for baseball stadiums), not a rule of the league/series. Of course, in basically ever professional racing series, overtaking of any kind is prohibited during a caution flag (either full course, or local).