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Another vote for proactively adding canonical tags to all pages. They're a great preventive measure in case someone else links with unusual parameters. Also, Google is "supposed" to understand the UTM tags and ignore them, but we've all seen cases where Google's actual processes don't work quite like they're supposed to. (I've seen plenty of utm-tagged URLs indexed in Google) (Plus, there's even less guarantee that other search engines would discount them and avoid dupe content. Bing et al may not be a huge traffic source on your site, but no sense throwing it away unnecessarily.) Paul

Thanks, everyone, for the responses. They were very helpful.