>
> (defun fn () '(1 2))
>
> (nconc (fn) '(3))
> -> (1 2 3)
>
> (nconc (fn) '(3))
> -> (1 2 3 3) ? should be (1 2 3)
>

Why do you think this is a bug?  The docstring of "nconc" says: 
"Concatenate any number of lists by _altering_ them.  Only the last 
argument is not altered, and need not be a list."  (append (fn) '(3)) 
always returns (1 2 3).

>
> If this is not a bug, then hfy-invisible, hfy-face-to-style-i may need 
> to be fixed.
>

Does the attached patch fix the bug you see?