So to the question: if you carry a firearm, should you know the laws you are to abide by. The answer is simple and really no different than for any other person: ignorance of the law is no excuse. If you carry a firearm, you should know the laws governing your right to carry. If you drive a car, you should know the laws governing the use of motor vehicles.
That being said, the gun laws in this country are a patchwork of varying restrictions, licensing schemes and permitting processes. Imagine if the same patchwork applied to the Second Amendment were to be applied to the First Amendment: you travel from Tennessee to DC, and in every state and county you enter a different set of laws dictates which bumper stickers are allowed on your car, your permit to worship is only valid at the Baptists churches and not the Methodist churches because the Methodists refuse to pay the state $200 religion registration fee every year, and at the political rally you are attending in DC you are not allowed to voice your opinion at all because DC doesn't allow the voicing of political opinions outside the home. Yeah, sounds pretty stupid but that is only a small sample of what gun ownership is like in this country.
To illustrate this stupidity, let's look at this map of the George Washington Parkway here in Northern Virginia:
|A felonious drive on the George Washington Parkway. Map courtesy of the Virginia Citizens Defense League.|
Trust me, it gets more stupid. In California some handguns are illegal simply because they are not the right color. In DC, Massachusetts, California and other states, simply owning the components that can make ammunition is illegal without the right permits. Why? Who knows, but I doubt gang-bangers spend their weekends at the reloading bench in their sheds pouring through reloading manuals and polishing cases with a tumbler. More amusing stories of this here and here.
So, did Duncan break the law? He sure did. But was the law he broke a rightful limitation of his rights? I think the answer to that is in the fact that DC dropped the charges against Duncan even though they seemed to have a clear-cut case. My bet is that DC looked at the situation -- a country preacher and veteran and handgun permit holder, accompanied by a federal agent and police officer, his family and grand children to an Independence Day celebration -- and quickly determined that there was likely no better sympathetic defendant to run up against in yet another case that would find its way to the Supreme Court (and remember, DC still owes Alan Gura quite a bit of money for Heller).