The same buff is overwritten from different classes. When doing instances as a bard, I'll see a cleric give us the +endurance buff, and if I give the fanfare for buff, it'll overwrite his, and his will overwrite mine if he recasts. It's not horrible as most classes have enough buffs that you can switch it up.... ie.. Bards have fanfares and can only choose one per bard, so if someone is providing +endurance, then use the fanfare of power, etc.