Returns: AX new file handle that duplicates the original
Info: Creates an additional file handle that refers to the same I/O
stream as an existing file handle.
Any motion of the file read/write pointer for one handle will
also affect that of the duplicate -- including any file read,
write, or file pointer motion via Fn 42H (Lseek).
The new handle inherits the Open Mode of the original.
Notes: This function has one main use in a single-tasking system: You
can close the duplicate handle, forcing DOS to FLUSH THE FILE
BUFFERS. This dup/close sequence is faster than closing and
re-opening a disk file. DOS 3.3+ offers a better method with
Fn 46H (redirect a handle to a different file/device)