In a video released by Matt Cutts, Google’s head of search spam, Matt said publishers who publish similar or duplicative stories on different URLs may use the rel=canonical tag to help consolidate the PageRank of the stories and avoid any issues with Google.
Matt Cutts did say that duplicate content won’t hurt you unless you are doing it for spam-related reasons. Cutts also did say that 25% of the web is duplicative, so you really don’t have to worry about it. But I guess in this case, to help with rankings, you may, as a publisher, want to use the rel=canonical tag to help Google know which is your primary page.
Of course, a site like this has dozens of articles on the topic of duplicate content. So should we use the canonical tag to point to one story or a category? Likely not.
For more on this topic, see our duplicate content category.