I believe it is not possible to obtain a patent on something if another company can show they've already been producing it for some time. So in your example, if no patent was applied before the first company began using the design, there is no way the second company could get exclusive rights to it.
I appreciate your comments, Mr. Tichbourne. A patent is indeed a tough thing to acquire, tougher to keep from being easily circumvented, and even tougher to pursue action on if it is directly violated. Doesn't Spyderco hold not only the "hole," but their pocket clip and serrations as well? But because these must be specifically defined in the patent terms, other companies freely produce knives with different hols, different clips, and different serration patterns. This can, of course, be viewed as a double-edged sword; imagine the monopoly a company might achieve if it could pattent something as vague as "serrations."
There is one thing that can be done to protect individuals' or companies' designs, however, besides legal action. The consumer can say no to knock-offs and ripoffs, and refuse to buy a copied knife. Yes, most of these copies are of such poor quality that the informed consumer will not be interested anyhow, but there are some knives out there that reproduce others' designs in passable quality, yet should nevertheless be boycotted.
There are several examples, but I am thinking specifically of Junglee Knives, especially their folder line. I once complained to the editor of a major knife mag after his publication reviewed a Junglee knife that was a clear rip-off. He responded that he was aware this knife and others he mentioned were stolen designs, and had complained to the company, but that much of a knife magazine's income comes from advertising for such items and he was effectively powerless. Make of that what you will, but we, the consumers, are not powerless. Don't buy copied knives!
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-Corduroy
(Why else would a bear want a pocket?)