Any particular country in mind? I'm thinking the US is a whole different thing from N-Korea in terms of how you might succeed, and sheer number of weapons to trigger.

I had the US in mind, but if another country might be easier to infiltrate, then by all means. I'd just like to know the option that leads to maximum destruction.

An army of cloned privates will have a devastating shortage of sergeants and officers. An army of cloned officers will be pretty useless. An army of cloned sergeants may function for a little while, but the complete lack of officers will eventually doom it. Officers, sergeants and privates are all necessary; officers provide the vision and the overall direction, sergeants are good at motivating their subordinates and leading small tactical engagements, and privates take orders and are willing to die for their flag. The point being that he absolutely does not have a well-coordinated army.

@AlexP All of the clones are created with his memories, including his plan which he came up with before he used the duplicator. So they all start perfectly on the same page. And all of them are well-trained and capable of leading, tactics, taking orders and dying, so I don't see why he'd have trouble filling all of those roles and creating a chain of command.

Relevant: The Simpsons on the shortcomings of an army-of-one: youtube.com/watch?v=HLITQXRH70M

"I don't see why he'd have trouble filling all of those roles and creating a chain of command": exactly because "all of the clones are created with his memories, including his plan which he came up with before he used the duplicator". Privates do not think like sergeants, sergeants do not think like lieutenants, lieutenants do not think like colonels and colonels do not think like generals. There is nothing worse than an army where privates believe that they understand what the generals think, except an army where generals think like privates.

@AlexP a perfect solider can be moved up and down the chain of command without issues. In modern armies, all officers are trained from the very beginning as rookies and capable of acting as privates (as opposed to old armies, where officers were coming as privileged class after purchasing a commission).

Does it have to be a car loaded with supplies coming through the gateway or can it be other things? Does it have to be something that can locomote itself through the gateway or can it be pushed or otherwise arrive with momentum?

Where did the duplicator come from?

it'd be easier to just drive a bunch of cars full of gasoline into an active volcano or something. Harder to detect, harder to stop after he's managed to dump 5 carloads a minute worth of CO2 into the atmosphere.

@Telastyn You know what's scary? The US alone is already producing so much CO2 that this wouldn't make a dent. It's comparable to hundreds of cars driving into volcanoes every second. I wasn't expecting this question to lead to my daily dose of climate dread, but here we are.

@Michael I don't want to get into how the duplicator works because it's complicated and prone to misinterpretation. But yes, it's only a gateway, so anything that comes out of it has to be able to move on its own power. That's why I assumed you'd want to use a car.

Is there a limit on how many copies the duplicator creates?

So... If you tilted the duplicator such that it was face-down, would anything exiting 4 hours later have been accelerating due to gravity for 4 hours?

@Separatrix Not intrinsically. The limiting factor is really that using the duplicator isn't legal, and you probably won't be able to create more than a few tens of thousands of duplicates before the government catches on.

Just put it somewhere no-one will find it, clone CO2 and wait until the Earth becomes uninhabitable. HA HA! Just kidding, no species would be so utterly dumb as to allow that to happen

@Chronocidal No, specifically the cars moving through the duplicator have been driving in circles for hours, coming out of one side of the gateway and into the other, over and over, through several parallel universes, until they arrive at yours. In the case you're describing, they'd fall out of the portal and have to climb back up over and over again. Needless extra effort, no breaking energy conservation.

Is the guy names Rick and does the army of Ricks setup some kind of large base? Maybe a citadel?

I'm quite sure there'll be a nuclear apocalypse situation some time before the black hole apocalypse. As long as he makes the duplicator inaccessible, he just needs to shoot a gun into it and the mass will eventually destroy the planet, create a new sun (does that count as nuclear apocalypse?) and, depending on the durability of the duplicator, create enough new mass for a black hole ... of course "eventually" is a relative term, so the faster you can get as much mass as possible out of it, the faster everything will go bananas.

@Morfildur Yes, yes, black hole and all that. Everyone wants to be clever. Trust me, I put thought into this duplicator, and I asked the question the way I did because I was sure that flooding the Earth with infinite mass wasn't going to work. Won't stop half the people here suggesting it, though.

I actually did the math when I wrote my answer, with the idea being to create a black hole by duplicating some heavy element like gold. Problem is, even gold would need to be packed in a radius from the sun to the earth to reach the schwartzchild radius, and the portal would be destroyed by intensive gravity long before it could form a singularity.