So they first started using duck.com for Internet search in 2009, while DDG had applied for registration of Duck Duck Go as a trademark in the field of Internet

So they first started using duck.com for Internet search in 2009, while DDG had applied for registration of Duck Duck Go as a trademark in the field of Internet search on April 2, 2008 (edit: USPTO does not give stable URLs - removed link). Sounds like a potential case of trademark infringement by Google, if Duck.com can be considered to be confusingly similar to "Duck Duck Go".