Are mirror sites (one site with 2 different urls but identical content) completely frowned upon and if so whats the best solution to resolve the problem - stick with one and whack a 301 redirect on the other?
If you have complete mirror sites then one will not be listed in search engines. Generally the oldest is listed, for instance if you have a .com and then set up a .co.uk to target the UK only listings, generally the .co.uk won't be listed.
Most people who want two domains with exactly the sme content will use a redirect of one form or another to save on the work of updating two sites.
The 301 permanent redirect is the safest redirect to use.