I did it for 17 minutes once, as I described earlier. I've had it happen many times for over 5 minutes. I never thought to count the number of complete 360 revolutions we had in the 17 minute one, but it had to be 75 or more I would guess...Part of the reason I pancaked was my arm was sore as hell...
two possible reasons why.
1: you both flew exactly the same turns with the same throttle, control input and turn radius through out the 17 mins. This opponent and you had a very rare and wonderfull fight becuase you are almost completely alike in style and aircraft.
2: that you both let the plane type determine the edge rather than individual style or risk commanding a quicker conclusion.
either way a long fight closely matched can be some of the best fun.
edit: a 3rd option i can see:
think of it like tennis. a very close match on deuce can swing advantage one way or the other for an extended time if both players score alternate points. You can win an inch and next turn lose an inch repeatedly. This is more likely than both parties flying exactly the same.