This is happening in Confluence 2.9.1, not sure if this is happening in older versions.

If I have the global theme set to Default Theme, http://localhost/dosearchsite.action works as it should when I do a search on test:
http://localhost/dosearchsite.action?queryString=test&where=conf_global&type=&lastModified=&contributor=&contributorUsername=

However, if the global theme is anything else (Left Navigation, Clickr or Confluence Classic), http://localhost/dosearchsite.action returns results with duplicate querystrings.
Searching for test results in this:
http://localhost/dosearchsite.action?queryString=test&queryString=test&where=conf_global&type=&lastModified=&contributor=&contributorUsername=