INT 21 - DOS 2+ - "DUP" - DUPLICATE FILE HANDLE
        AH = 45h
        BX = file handle
Return: CF clear if successful
            AX = new handle
        CF set on error
            AX = error code (04h,06h) (see AH=59h)
Notes:  moving file pointer for either handle will also move it for the other,
          because both will refer to the same system file table
        for DOS versions prior to 3.3, file writes may be forced to disk by
          duplicating the file handle and closing the duplicate
SeeAlso: AH=46h