A bungled robbery NEVER results in a double tap to the back. In a bungled robbery, the gun goes off while the victim and perp face each other most often, and there is not a second shot to confirm death. That's the whole point, the perp panics and forgets to take the stuff - he doesn't calmly put another round through the heart.
Seth Rich was alive and conscious when the police arrived. He died an hour later. Doesn't sound like a shot through the heart to me.
I checked the last two executions by firing squad in Utah - four bullets, not two, admittedly, but that involves aiming at the heart. The most recent was pronounced dead within two minutes. The next recent was dead within four minutes. Note this is death - not unconsciousness. Unconsciousness should happen sooner.
A Straight Dope discussion seems to give several examples of unconsciousness happening within seconds.