When trying to change the web app authentication from Windows(only) to claims based I encountered this error during the removal of the existing web app.
‘An object of type…… “Unprovisioning SharePoint – 80” already exists under the parent….”
I wanted to keep things neat and tidy and did not want to create yet another new web app with a new name etc.
After hunting around for a solution I came across a ‘hung’ timer job relating to this. Deleted this ‘culprit’ timer job definition and tried to remove the old/existing web app….and voila!! It worked!!