Individual SharePoint documents are always restored to SharePoint workspaces as checked out to the credentials specified by the logon account used for the restore. The documents must be checked in or published by that user before others can use them.
If you try to restore over a document that is published or checked in, the restore will fail. If you try to restore over a document that is checked out, the restore will fail if the document is checked out to a user that differs from the logon account credentials used for the restore.
Previous versions of individual documents that existed when the documents were backed up are not preserved when they are restored into a workspace, and the document versions are reset. The only way to preserve version history for documents is to restore the entire SharePoint Portal Server database.
Documents that exist in the target location may be overwritten by the restore depending on the overwrite properties for the restore job. If an existing document is overwritten, then the version history for that document is lost as well.