Molten Strike + Iron Grip...does it work?

Does a setup with both Iron Grip as passive and the gem effect an attack twice?

I'm constantly theory building my Marauder to a very high str setup, but not sure of mainskill yet.

If it double dips, I'll probably do:
Lightning Strike - Multistrike - Iron Grip - Physical to Lightning - Faster Attacks - Life Leech

with very high str gear and maybe dual Doryani's.

Or same setup with Moltenstrike - MS - IG - AddFire - FA - LL

Or Pillar.

The gem does not stack with the passive. It's still the same effect. You're better off just using the passive with a different gem. Also, don't focus too much on str for damage, it's a very low bonus, and actual damage passives will always give a better benefit for less points.