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Don't know if they still test these as an initial dupe sniff test, but I'd stay away from them if at all possible. Best practice a few years ago was that it was better to do without a description rather than have dupes.
For example on user generated pages like forum posts I just use the sites primary meta description since the user is not asked to enter one when they create a forum topic.
B Webmaster tools specifically flag a "missing description" and "description too long or too short" as "HIGH severity" SEO blunders under their SEO report. Since a lot of other factors appear similar to G's and G doesn't specify one way or the other, beware.