I think there should be a limit as to buffing stats and I am against duration time stacking as it takes away skill from the game and leaves the same problems. Two privateers casting the same spell on the same turn should render the first cast ineffective, and the same for all other duplicate casts. Skills should top out at 100% buff, or even 150% to 200% but there should be a limit.
I'm curious how you know that two of the same buff are stacking, because I want to verify that.
It would be good to know that when two El Toro's do the same Accuracy buff that the second didn't do it for naught.
One way to check is to be in a battle with a friend (that is, someone on your friends list). If you inspect their character through the friends list, the stats that you see are boosted by what has happened in the battle.
I do think the way the buffs stack is fine. You have the choice of being super powerful for a few turns, or having moderate extra power over many turns.
Also, having identical buffs cancel out previous ones would make it so that if two people cast the same thing on the same turn, one of them would be totally wasted. Considering how tricky it is to communicate in a battle, I know I would get frustrated.