Anyone who knows how to use a machine shop can duplicate any key. For most keys there are specific machines that don't require skill to operate that duplicate it. The skill required to duplicate the other keys, combined with the "do not duplicate" stamp on the other keys means that such keys are rarely duplicated.

In the vast majority of the US, there is no law against businesses duplicating keys stamped with "do not duplicate" which means it's usually not too hard to find a shop that will do so.

That depends. You are legally correct. Most shops when seeing a do not duplicate key will do some checking to see if you should be allowed to duplicate the key (but there is no standard here), compared to any other key which they will duplicate no questions asked.

Also some "secure" keys don't have any blanks. Most shops don't have the ability to duplicate them. You can still get them copied, but you need a skilled machinist at considerably more cost.