A particular website is usually referred by the domain
example.com, which is also the one which is promoted and used everywhere. But there's also
example.net (in real, the hyphen in the domain name is removed), which serves the exact same content, including the links and references to images, style information etc.
I usually oppose to these kind of setups and recommend a 301 Permanent Redirect for reasons of unambiguity, simplicity and single content.
I'm a fan of unambiguity because my background server administration and this is a very important principle, but it's only relevant on the low levels of digital communication. Are there any other reasons, from the webmaster's and user's view, to oppose to such setups aside from duplicate content in SEO?