A DUPLICATE URL REFERENCE SERVING THE SAME CONTENT IS NOT DUPLICATE CONTENT! MULTIPLE URLS TO THE SAME CANONICAL CONTENT ARE SIMPLY DIFFERENT PATHS TO THE SAME RESULT.
Duplicate content is multiple canonical urls with the exact same content, and is NOT two different paths to the same canonical url! This is a simple, but critical fact of what we are discussing. Does a url with a 301 redirect to a 2nd url mean that the content is duplicated? No!
Pay attention to this: "all refer to the same article"
And I have to believe that the G development staff and Matt Cutts clearly understood this when they addressed how canonical content is now handled by G in its indexing process.
The key here is that you should be consistent with internal links within your own site, which makes perfect sense. However, you have no control over external links and/or how a user may enter a url to get to a certain url/page on your site. Therefore, the internal consistency is what is important, not how some external link or user url entry may be used to get to the canonical target url to a particular content. G does not see or care what the user may enter, and if an external link can be resolved, by the spider or by the web server as it delivers the targeted content, G really does not care. What it looks at and indexes is the final resolved url that actually delivers the content.
I realize that this was not always true. This was a major point of contention, and the reason 302 hijacks worked, and why many dup content penalties used to occur, even if the webmaster was totally clean, and not at fault. One of the major goals of G's BD and other canonicalization development efforts was to eliminate the external causes of this kind of penalty, and to rely on the final canonical url 99.5% of the time, per Matt's blog.
The flaw is NOT in how a particular web server delivers content. The flaw was how G used to handle it. G, according to Matt's posts on canonicalization, and according to the results I see on my own site and many others, has fixed this. However, many webmasters seem to want to hang on to the old problems of the past, and not accept that things change. What used to be true, in this case, no longer applies, at least as far as uncontrollable external factors are concerned. How we implement links internal to our own sites probably does still matter, but how a user or external link refers to a url no longer does.
[edited by: RonnieG at 4:47 am (utc) on Nov. 14, 2006]