When you try to create a Document Workspace for a document in a Document Library in SharePoint, you may see the error:
"This document is already stored in a Document Workspace. A new Document Workspace cannot be created for this document."
The error can be a little misleading. It can occur when you try to create a Document Workspace *from* a Document Workspace. In other words, if a site was created as a Document Workspace, then you won’t be able to create Document Workspaces for any of the documents there.
Users may add new Document Libraries to a Document Workspace (and perhaps forget that it was a Document Workspace in the first place) and then try to create a Document Workspace for one of those documents. One way to check for this is to go into Site Content And Structure (_layouts/sitemanager.aspx) and hover your cursor over the icon for each site. Document Workspaces will display "Document Workspace" instead of "Site" or "SharePoint Site".