Overleaf citation multiply defined with A&A class file

Question

In Overleaf, when citing a reference within the document class aa from the A&A Latex package (from the journal Astronomy and Astrophysics) and compiling with TexLive2021, I get a warning Package natbib Warning: Citation 'knuth-acp' multiply defined. However, the bib file only contains one entry, so the label should not be defined multiply.

The warning does not appear with the document class article and/or compiling with TexLive2020. However, I need TexLive2021 and the document class aa for another project (TexLive2021 because I require a new version of the siunitx package, which is not present in TexLive2020).

I know I can ignore the warning, but in principle this can disguise warnings about labels that are actually defined multiply.

What is the reason for the warnings and how can I fix them?

The minimal working examples (MWEs) are:

---

MWE (Overleaf): https://www.overleaf.com/read/kqkzzqphzqyv

---

MWE (direct):

main.tex (tex file):

%\documentclass{article}
\documentclass[letter]{aa}

\usepackage{natbib}

\begin{document}

\cite{knuth-acp}

\bibliographystyle{aa}
\bibliography{lit_test}

\end{document}

lit_test.bib (bib file):

@book{knuth-acp,
   author = "Donald E. Knuth",
   publisher = "Addison-Wesley",
   title = "The Art of Computer Programming",
   series = "Four volumes",
   year = "1968",
   note = "Seven volumes planned",
   keywords  = "knuth,programming"
}

aa.bst: A&A Latex package (file too long to paste)

aa.cls: A&A Latex package (file too long to paste)

Answer 1

As the OP pointed out, switching to TexLive2020 in the overleaf configuration is a quick workaround, for those who do not need TexLive2021. Thx.