I don't knowingly make an exact duplicate of someone else's card for a few reasons:
- I think it is almost impossible for me to exactly duplicate another card. Stamps stamp differently, I don't have the right color paper, I dislike the colors they chose, etc.
- Half the fun is playing around with layout and design elements. My cards almost never turn out the way I envisioned them.
I think the only way around this is for people to say what they want done with their ideas, something along the lines of the creative commons license. That way people like me can say "do whatever you want with it" and people who are professionals and want reimbursement can say that. I expect the whole thing would be on the honor system since there really isn't any way to know someone copied your idea or if they came up with it independently.
If someone copied one of my cards I would be flattered. If they actually charged money for one of my cards I would be shocked (and maybe open an eBay account and do the same). But one of the things that attracted me to stamping was what I perceived as a more laid back, willing to share attitude among hobbyists, and I would be saddened to find that untrue.