"The item already exists" when adding trying to add files to TFS

Clean the Cache folder on client computer. The folder path is: C:\Users\username\AppData\Local\Microsoft\Team Foundation\６.0\Cache.

Use the tf destroy command to destroy, or permanently delete, version-controlled files from Team Foundation version control. Check:https://msdn.microsoft.com/en-us/library/bb386005(v=vs.100).aspx

This was somewhat related to Cece's answer in that even though it was deleted on the server, my local computer still thought the file were in the uploaded workspace.

What I did was create a temp folder TempRepo on my desktop and moved the files from the "classic" folder to it. I mapped my workspace to this TempRepo folder. Then I was able to "add existing files" to the workspace and push my files from this temp folder to Visual Studio Online.

When I was done, I just mapped the workspace back to the original folder and performed a "Get" operation. Now I have all the files uploaded onto TFS and the correct workspace set.