deadsea - 12:04 pm on Apr 26, 2011 (gmt 0)

enigma1: parameter order is just one possible way that multiple urls get generated with the same content. Other issues that I've seen:
1) Extra tracking parameters for ads, affiliates, marketing, or sessions
2) Extra parameters that subtly change the page such as adjusting the breadcrumbs based on where you have been on the site, turning off particular teasers, or adjusting the color of a product.
3) Url capitalization. /hello.html and /Hello.html are not the same url but often return the same page.
4) www.example.com vs example.com
5) Merged data such as two product ids that are for the same product and handled via an internal forward rather than a redirect.
6) Extra slashes in the url. Apache has a habit of serving the same page for /hello.html and //////hello.html
7) default documents: /index.html vs /
8) a beta or development version of the site that accidentally goes live on a different host or subdomain