<File> is already checked out by <user>.  Continue?

This warning has the following cause and solution:

• You attempted an operation on a VSS file that is checked out by another user.

  VSS gives you the option to continue, which means any differences between the changes you make to the file and the changes made by the other user will need to be merged in the future.  Click Yes to continue your operation and create a multiple-checkout situation; click No to cancel the check out.