DOS Fn 45H: Duplicate a File Handle

                                                         Compatibility: 2.0+ 
 Expects: AH    45H
          BX    existing file handle
 Returns: AX    new file handle that duplicates the original
                error code if CF is set to CY
    Info: Creates an additional file handle that refers to the same I/O
          stream as an existing file handle.

          Any motion of the file read/write pointer for one handle will
          also affect that of the duplicate -- including any file read,
          write, or file pointer motion via Fn 42H (Lseek).

          The new handle inherits the Open Mode of the original.

   Notes: ■ This function has one main use in a single-tasking system: You
            can close the duplicate handle, forcing DOS to FLUSH THE FILE
            BUFFERS.  This dup/close sequence is faster than closing and
            re-opening a disk file.  DOS 3.3+ offers a better method with
            Fn 68H (Commit).

See Also: Handle-Oriented File I/O
          Fn 46H (redirect a handle to a different file/device)
          Standard I/O
          DOS Functions