LaTeX: possible duplicated footnote in PDF from object signatures if with intersphinx and latex_show_urls='footnote'

[jfbu]

Describe the bug

PDF output may contain a duplicated footnote showing URL if intersphinx and latex_show_urls='footnote' are both used.

How to Reproduce

index.rst:

.. highlight:: rst

======
 TEST
======

The C Domain
============

.. c:function:: PyObject *PyType_GenericAlloc(PyTypeObject *type, Py_ssize_t nitems)

   bla

conf.py:

master_doc = 'index'
project = 'Test LaTeX warning bout footnote multiply defined'

extensions = [
    'sphinx.ext.intersphinx',
]

latex_show_urls = 'footnote'

intersphinx_mapping = {
    'python': ('https://docs.python.org/3/', None),
}

Then execute make latexpdf. The console output will show near end:

Latexmk: ====Undefined refs and citations with line #s in .tex file:
  Label `0.footnote.1' multiply defined
Latexmk: All targets (testlatexwarningboutfootnotemultiplydefined.pdf) are up-to-date

and the PDF is indeed with a double footnote 1:

Environment Information

Sphinx version: 8.3.0+/2b7e3adf2
Python version: 3.13.3 (CPython)
Docutils version: 0.21.2 
Pygments version: 2.19.1
Jinja2 version: 3.1.6

Sphinx extensions

['sphinx.ext.intersphinx']

Additional context

The LaTeX mark-up shows only once the footnote.

\begin{savenotes}\begin{fulllineitems}
\phantomsection\label{\detokenize{index:c.PyType_GenericAlloc}}
\pysigstartsignatures
\pysigstartmultiline
\pysiglinewithargsret
{\sphinxhref{https://docs.python.org/3/c-api/structures.html\#c.PyObject}{\DUrole{n}{PyObject}}%
\begin{footnote}[1]\sphinxAtStartFootnote
\sphinxnolinkurl{https://docs.python.org/3/c-api/structures.html\#c.PyObject}
%
\end{footnote}\DUrole{w}{ }\DUrole{p}{*}\sphinxbfcode{\sphinxupquote{\DUrole{n}{PyType\_GenericAlloc}}}}
{\sphinxhref{https://docs.python.org/3/c-api/type.html\#c.PyTypeObject}{\DUrole{n}{PyTypeObject}}%
\begin{footnote}[2]\sphinxAtStartFootnote
\sphinxnolinkurl{https://docs.python.org/3/c-api/type.html\#c.PyTypeObject}
%
\end{footnote}\DUrole{w}{ }\DUrole{p}{*}\DUrole{n}{type}\sphinxparamcomma \sphinxhref{https://docs.python.org/3/c-api/intro.html\#c.Py\_ssize\_t}{\DUrole{n}{Py\_ssize\_t}}%
\begin{footnote}[3]\sphinxAtStartFootnote
\sphinxnolinkurl{https://docs.python.org/3/c-api/intro.html\#c.Py\_ssize\_t}
%
\end{footnote}\DUrole{w}{ }\DUrole{n}{nitems}}
{}
\pysigstopmultiline
\pysigstopsignatures
\sphinxAtStartPar
bla

\end{fulllineitems}\end{savenotes}

so the the bug is in the interactions of savenotex, fulllineitems, footnote LaTeX environments and LaTeX commands such as \pysiglinewithargsret.

The problem shows in our own PDF docs currently at bottom of page 166:

[jfbu]

This bug reveals the inadequacy of a somewhat hasty work-around added at a1d9127, as part of #10191. I can not currently test easily if that work-around ever worked to start with ¹, but I doubt it as it seems all it did was to hide a symptom, but the PDF probably had the duplicated footnote already (as we can see in our own docs at page 166). The symptom of LaTeX warning on the command line can only be seen with the very recently released LaTeX 2025-06-01 which corrected a 30 years old typo in amsmath. The famous "butterfly effect" of fame at the height of dynamical systems theory. The fix of that typo activated a modification decided by amsmath of a core LaTeX macro and the result is that now LaTeX complains at end of build, which is good as it helped realize about these duplicated footnotes in some cases. So, thanks to LaTeX people to have made that change...

Update: somewhat miraculously it proved possible to fix this with minimal effort.

Footnotes

1. sphinx.pdf from November 30, 2022, as recovered from Wayback machine already has the duplicated footnote at bottom of its page 80. ↩