Problem:

My Team Build keeps failing with an error that my source folder is already mapped in another workspace. The path C:\Builds\1\Demo\Reports\Sources is already mapped in workspace 7_1_WIN-GS9GMUJITS8.

Solution:

Use the tf.exe tool to delete the workspace holding on to that location.

Explanation:

tf workspace /delete [/collection:TeamProjectCollectionUrl] workspacename[;workspaceowner] [/login:username,[password]]