You may come across this error while publishing WF created using Nintex. Most likely, You exported your workflow from one environment and importing it into a site in another environment. You cold import and save it successfully but when you try to publish, you get – “Item has already been added. Key in dictionary: vti_privatelistexempt” key being added:””
This normally happens if the imported workflow has ‘Request Data’ action and is using InfoPath as default form type. When WF was imported in new site, Infopath (.xsn) file corresponding to that Request data action was also added in ‘InfoPath’ folder in hidden ‘NintexWorkflows’ library. While publishing, Nintex was trying to add infopath form to the same folder again and thats when it was throwing this error.
Resolution is – from the new site, delete the imported infopath file from ‘InfoPath’ folder in ‘NintexWorkflows’ library and publish again. This will let Nintex generate the new & correct infopath .xsn file in ‘NintexWorkflows’ library.