INT 21,45 - Duplicate File Handle

	AH = 45h
	BX = file handle

	on return:
	AX = new file handle if CF not set
	   = error code if CF set  (see DOS ERROR CODES)

	- gets another file handle for the same file
	- both file handles move in unison
	- often used to flush file data and update a file directory
	  entry without closing the initial file