What everyone else said sounds right, with one caveat.
If the initial command to kill the charm caster allowed a second save against the domination spell, I would not grant a third one when the charm spell is cast.
I think what Udalrich means if the target of the kill command was already someone that the creature under the dominate wouldn't kill and would have received a saving throw for then a 3rd saving throw for Charm wouldn't have been appropriate.
I could be misunderstanding that though