Int 21 Fn 45  - DOS 2+ - "dup" - Duplicate File Handle                     [D]

   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

See Also: AH=3Dh,AH=46h