It is generally understood that makers find inspiration in one anothers work. It is also understood that new ideas are often born of old ones, and that so many variations of so many styles have already been done it's probably quite difficult to come up with something completely unique.
That said, there remains a huge difference between finding inspiration and copying, almost exactly, another maker's work.
A near exact copy is theft. It is theft because the copier receives money for another man's product.
How does one differentiate between an inspired design and a copy? Well, most of us just know it when we see it.
Neil Blackwood's credit card knife is a perfect example. A maker may look at that design and say "hey, that's pretty clever. I like the idea of making a knife that can be concealed as a credit card. I wonder if I can make a better one by maybe changing the way it's held, the shape of the edge, the material, and the appearance." A thief, on the other hand, might look at same knife and say "I bet I can make a lot of money off that person's design."
It's wrong to copy another maker. I would imagine that those who do copy, or steal others' designs know that it's wrong when they do it.
The solution? Quite simply, makers should not intentionally copy another maker's work. Buyers should not buy what they know to copies of another maker's work. Further, makers would be wise to document their work and speak out when it appears that that work has been copied or a design stolen.
That's about it though. Applying for patents or trademarks probably isn't worth the trouble in most cases. Simply doing things the right way and defending oneself is probably the best method. It won't prevent some people from behaving improperly or unethically, but then again, nothing ever does...