Question about ignite

So for reference. Ignite that i use has 3 methods of application.
1. Damage from fire attacks/spells
2. Flame wall which places the ignite icon on enemies for its burning damage and lingering effect.
3. Infernal legion which acts "as hit by" in an area around the minion

All these instances alone place the ignite debuff icon on the enemy.

My question is this.

How does stacking or overwriting happen?

Is it first come first served until timeout?
Is it strongest wins out base on damage per second?
Is is newest always overrides?


Secondary question.

In regards to infernal legion. I imagine that no passive nodes on the tree will affect it?