I would most recommend simply buying a new copy, it will only be 5 or 10 bucks. If you're only playing in single player mode you don't need to worry about duplicate keys on old used games. Another option if you already have paid for the game is to crack the copy protection, usually pretty easy for an old game like quake. If you don't know how to use a debugger you can usually find a keygen and/or a cracked executable, which will typically come with a torrent release. In this case I highly recommend antivirus because many keygens and cracked releases come with nasties. Legally, it's a gray area, you already own a license to the game but it's usually illegal to copy or crack it even so. Morally I don't see a big problem.
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"Prohibition will work great injury to the cause of temperance. It is a species of intemperance within itself, for it goes beyond the bounds of reason in that it attempts to control a man's appetite by legislation, and makes a crime out of things that are not crimes. A Prohibition law strikes a blow at the very principles upon which our government was founded." --Abraham Lincoln
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