I try and explain to clients who bring in other peoples tattoos, that it's not cool to get someone's exact tattoo. Usually saying something along the lines of "how bummed would you be if you saw someone wearing your exact tattoo, pretty bummed out right, why would you wear someone else's tattoo?" I think it is disrespectful to the client with the original, and especially to the tattooer who busted his ass to draw and excite the tattoo. Do your research find reference, but make every piece one of a kind.