I think there's a gray area where a copy gets close enough to the original that it's clear what the maker is trying to do. But the beginning of that gray area is hard to find. Especially since the Internet allows us to search a world's worth of images and information, you're going to encounter what they call "convergent design" eventually. The human hand has shapes it likes to hold, and there are a limited number of things you can do with a knife blade.
The end of it, where a copy has the original's trademark (an engraving or a specific shield in the case of traditionals), is sure easy to see! When you can buy a $70.00 "XM" from a certain website, with the flaming horse head silkscreened onto the blade, there is no doubt what's going on.
It's also important to define the difference between a trademark and a "pattern". The words "sodbuster", "trapper", "canoe", "congress", "electrician's knife", all refer to a general pattern of knife, but there can be many variations on the theme within that pattern. To the best of my knowledge, nobody owns the word "sodbuster", although I expect somebody to try.
Edit: There is also an existential question about the limits of imitation, and "becoming the mask". Which is to say, if a copy is so good that it cannot be distinguished from the original, is it still a copy?
I suppose that the answer would be "yes", if the maker personally remembered each knife he had made. Otherwise, it's an interesting discussion for another thread.