Buffs - how do they stack?

Silberklaue - Heavens Tear

I've heard buffs don't stack. if a buff is cast on a char who already had the same buff, but from a higher lvl char, the better buff is deleted.

is that correct (and do you understand what I'm trying to say -.-)?

I don't know much about buffs, but when different buffs which have the same effect (better defense or something) from different classes are cast on one char, do they stack?

are there even different buffs with the same effect?

I hope it's not too confusing what I've written...

Metraze - Sanctuary

I've heard that too, though never tested it myself. I do know that Ironheart's stack - so does Thunderball (DoT) ... my guess is anything that lasts seconds will stack. Anything that lasts minutes will replace anything that uses the same status icon.

For example:
Cast Celestial Guardians Seal (lasts 30 min)
Use Life Powder (lasts 10 min)

after 10 min, the Life Powder wears off, you still have the +MP/sec from the seal - but no +HP/sec ...

Klath - Sanctuary

Depending on buff, the last one will overwrite the first.
Magic shell is the worse here, it will never be updated, so if you casted a 30 min version, once passed the 30 mins, it will be over, no matter if you casted again the same 30 min version or the 1 hour version. All other cleric buffs seen to refresh when recast.
Those apothecary shops will be overwritten and will overwrite cleric buffs, so beware before buffing someone who got one of them, they might get mad at you.