I remember hearing somewhere that if you visualized the IPV4 address space (32 bits) as being the size of postage stamp, IPV6 (128 bits) is the size of our solar system. Generating a dupe is just not going to happen.
Also, if anything you're more likely to get a duplicate the same machine than two different machines because most Guid generation algorithms will embed your computers NIC MAC address within the guid (it's 48 bits). Although there are algorithms that don't embed the MAC address and are just purely random. see: http://guid.us/
Edit: another fun scale example
the volume of the earth is roughly 10^27 cubic centimeters. meaning that every cubic centimeter for the ENTIRE VOLUME of the earth could have 340,000,000,000 guids all to itself. this number is mindbogglingly big.
Alternatively, every square NANOmeter of the surface of the earth could have roughly 650,000 guids all to itself.
