I guess it can be considered an Emacs bug, yes.

On Wed, Mar 6, 2024, at 1:15 AM, Arash Esbati wrote:
> Radon Rosborough <radon.neon@gmail.com> writes:
> 
> > When I perform a fresh build of AUCTeX from Git, I get the following
> > code as part of tex-site.el:
> >
> >     (defvar TeX-lisp-directory
> >       (expand-file-name "auctex" (file-name-directory load-file-name))
> >       "The directory where most of the AUCTeX lisp files are located.
> >     For the location of lisp files associated with
> >     styles, see the variables TeX-style-* (hand-generated lisp) and
> >     TeX-auto-* (automatically generated lisp).")
> >
> >     (add-to-list 'load-path TeX-lisp-directory)
> >
> > This is problematic. As the documentation for `load-path' states:
> >
> >     Use ‘directory-file-name’ when adding items to this path.
> >
> > But `TeX-lisp-directory' has a trailing slash. This seems like a bug
> > to me. Can it be fixed?
> >
> > The reason this came up is that I maintain an Emacs package manager,
> > straight.el. Once straight.el builds a package, it adds the directory
> > to `load-path', using `directory-file-name' as specified by the
> > documentation of `load-path'. But since AUCTeX insists on also adding
> > its own entry to `load-path', and does so with a superfluous trailing
> > slash, we end up with two duplicate entries. Under some
> > configurations, this causes duplicate entries to appear when using M-x
> > find-library.
> 
> (I'm going through old bug reports that unfortunately weren't resolved
> at the time.)
> 
> Is this issue still present?  Looking at the docstring:
> 
> ,----[ C-h v load-path RET ]
> | load-path is a variable defined in ‘C source code’.
> | 
> | Its value is shown below.
> | 
> | List of directories to search for files to load.
> | Each element is a string (directory file name) or nil (meaning
> | ‘default-directory’).
> | This list is consulted by the ‘require’ function.
> | Initialized during startup as described in Info node ‘(elisp)Library Search’.
> | Use ‘directory-file-name’ when adding items to this path.  However, Lisp
> | programs that process this list should tolerate directories both with
> | and without trailing slashes.
> | 
> |   This variable may be risky if used as a file-local variable.
> |   Probably introduced at or before Emacs version 1.12.
> | 
> `----
> 
> You're correct about `directory-file-name', but the version with
> trailing slash is also tolerated.  If `find-library' is confused by
> "/foo/bar" and "/foo/bar/", isn't this more an Emacs issue?
> 
> Best, Arash
> 
> P.S. I also added another mail address of yours to this message.  Sorry
> if you get this message twice.
>