The only thing that caught my eye was Google redirecting duck.com to google.com when DDG expressed interest in purchasing the domain. That seems a little underhanded, but not illegal....
Duck Duck Go has a TM application filed on April 2, 2008. Google acquired duck.com's owner in 2010.
There is a very reasonable case for a WIPO domain dispute, which isn't particularly costly (granted if Google lost, and then sued you it would be expensive.) Very good way to generate a lot of press coverage.
For any sort of mark dispute, you shouldn't go by when Google acquired On2, you have to go by when On2 acquired the name. I assume this was closer to the domain creation date of 1995 (or the On2.com creation date of 1999).