Debuffs (and buffs) - which ones stack, do they extend?

Is there a reference (preferably something official/authoritative) showing which debuffs stack? (i.e. which ones I can have multiple of the same item on one character).
And likewise for buffs.

It would also help to have some kind of rationale so we can guess which ones will/won't stack.

e.g., Damage over time stacks (can have multiple), but burning (which also does damage over time) does not. (Seems strange).
There are some others which stack, but not sure what they all are.
(And going forward would be nice to know about new buffs/debuffs.)

Does hitting somebody with a duplicate, non-stacking buff/debuff of longer duration extend the time that the buff/debuff is in effect?
e.g., if they stunned for 1 turn, and I hit them with a stun for 2 turns. What happens, does the stun get extended to 2 turns? Or does the initial stun prevent the later stun from landing, so they are still just stunned for 1 turn.