INT 21 - DOS 2+ - "DUP2", "FORCEDUP" - FORCE DUPLICATE FILE HANDLE
        AH = 46h
        BX = file handle
        CX = file handle to become duplicate of first handle
Return: CF clear if successful
        CF set on error
            AX = error code (04h,06h) (see AH=59h)
Notes:  closes file with handle CX if it is still open
        DOS 3.30 hangs if BX=CX on entry
        moving file pointer for either handle will also move it for the other,
          because both will refer to the same system file table
SeeAlso: AH=45h