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INT 21 - DOS 2+ - FORCE DUPLICATE HANDLE (FORCDUP,DUP2)
        AH = 46h
        BX = existing file handle
        CX = new file handle
Return: CF set on error
            AX = error code (04h,06h) (see AH=59h)
        CF clear if successful
            AX = new file handle (undocumented)
Notes:  closes file with handle BX 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=3Dh,AH=45h