Duplication Detector is a Toolserver tool created for
Wikipedia:Copyright problems on the English Wikipedia. It compares any
two web pages to identify text which has been copied from one to the
other. Either, neither, or both pages may be current or old revisions
of a Wikipedia article. For more information, see:

http://toolserver.org/~dcoetzee/duplicationdetector/

The PHP source for Duplication Detector is available under the
Simplified (2-clause) BSD License. It does not require Toolserver to
run, so feel free to download and use it yourself using your own
webserver or the php command-line tool.