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Agreed - this is generally an issue with relative paths, and job one is to fix it. In most cases, you really don't want these crawled at all. I do think rel=canonical is a good bet here - 301 redirects can get really tricky with http/https, and you can end up creating loops. It can be done right, but it's also easy to screw up, in my experience.

If the webserver does not support (or the admin does not want to enable) this feature you could always have your frontend-templates have a small string wich holds the date/time when the page was last updated. Something along the lines "last updated on: ...." at the bottom or top of the content area. It's also an useful bit of information for users.