How will ignore elemental resist and ele penetration stack?

If you hit harder you'll get more dmg back. But you won't ignore your own resistances.

Reflect is based on actual damage you deal to an enemy. You get hammered harder, but only because you are dealing more damage in the first place, not because the ignore resist effect transfers. The ignore resist effect does not transfer.

Ex. 1200 fire damage, enemy has 50% fire resist, and reflects 10% of elemental damage. You have 75% fire resistance.

Before: 600 fire damage dealt. 60 fire damage is reflected onto you. You take 15 fire damage.

After: 1200 fire damage dealt. 120 fire damage is reflected onto you. You take 30 fire damage.