Compatibility: 2.0+
Expects: AH    45H
BX    existing file handle
                                                                  
Returns: AX    new file handle that duplicates the original
error code if CF is set to CY
                                                                  
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 68H (Commit).

Fn 46H (redirect a handle to a different file/device)
- -

DOS Fn 45H: Duplicate a File Handle