Int 21H, AH=46H

2 and higher

“DUP2”, “FORCEDUP” - FORCE DUPLICATE FILE HANDLE

AH = 46h
BX = file handle
CX = file handle to become duplicate of first handle

CF clear if successful

CF set on error
    AX = error code (04h,06h) (see #01680 at AH=59h/BX=0000h)

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
        

AH=3Dh,AH=45h

Text based on Ralf Brown Interrupt List Release 61