Yes, there are definitely lots of other ways to create duplicate ways of hitting the same content. However, there are at least two important (IMHO!) things to keep in mind:

1. Appending parameters to the end of a URL is categorically different from appending what look like directories to the end of a URL. It is a no-brainer to identify parameters appended to a URL (anything that starts with “?”), and many search engines have done this for a long time — e.g., automatically ignoring PHP session IDs.
2. When we’re talking about directories, it’s pretty hard to refute the empirical evidence that Google does put this type of ‘duplicate’ content in their supplemental index.

As an example of the latter, I can tell with a quick check at Google right now that planetozh.com has about 7,230 pages in the Google supplemental index. (Given their recent announcement to drop the supplemental label, and to drop support for the most common types of search queries designed to return supplemental pages, there’s no telling how much longer this information might remain available.) If you look through them, you’ll see that many of your 7 thousand pages of supplemental content are in a /bookmark/ or /tags/ directory, while others are in date-based archives containing text that is also available via individual article permalinks.

In other words, while I’m not too worried about random people appending random parameters to URLs, I am worried about the external world seeing a whole bunch of structurally different URLs, apparently referencing different directories, all containing the same content. That is what is happening with your blog right now, and unfortunately the “smart people at Google” emphatically do consider this a duplicate content issue.

(By the way, you’ll see lots of supplemental pages for this site, too, as a result of a change in my permalink structure awhile back; fortunately, all the older pages which Google thinks are the definitive versions are 301-ed to the new versions.)

All the best,
Greg

I would be really surprised if the smart people at Google would consider this a duplicate content issue.