I suspect that people are more concerned here with duplicate links going to the same places, rather than spider friendly URLs? Spiders have no problem with even quite complex dynamic URLs these days. The issue is duplicate content, or the
appearance of duplicate content.
Contrary to popular belief there is no
penalty for duplicate content. All that will happen is that Google and the SEs will chose the URL they feel is most appropriate. But this is where the issue becomes a potential problem. More for user friendliness than for search engine friendliness, IMHO.
From a user friendly point of view, it is best if the same url is shown for each unique page of content. That way when users email their friends the url of their gallery, (for example) it is the same URL each time - everyone is
'on the same page'.
As a byproduct of this, a single URL for each unique viewable piece of content will also help the search engines to spider a site and assign the correct level of 'ranking' for that content - which is also a usability issue, rather than a search engine 'promotional' issue IMHO.
As for the technical issues involved to fix this, I haven't got a clue. I can't even hand code HTML.
My 2 cents.