Different equations with the same tag cause wrong reference

Question

I would like to achieve something quite similar to this question, except that I would also like to reference said equations. Here's a minimal example:

\documentclass[12pt]{article}
\usepackage[hidelinks]{hyperref}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{mathtools}

\begin{document}
\section*{Chapter A}
\begin{equation}\label{star1}\tag{$\star$}
    a^2 + b^2 = c^2
\end{equation}
The main equation of this chapter is the Pythagorean theorem, \eqref{star1}.

\pagebreak

\section*{Chapter B}
\begin{equation}\label{star2}\tag{$\star$}
    i^2 = -1
\end{equation}
The main equation of this chapter is the definition of the imaginary unit, \eqref{star2}.
\end{document}

The problem is that \eqref{star2} points to the Pythagorean. I also get this warning which I am pretty sure is relevant: destination with the same identifier (name{equation.0.1}) has been already used, duplicate ignored <to be read again>

Thanks in advance.

Answer 1

The package hyperref should be loaded last: only a few packages have to be loaded after it and none of those in the MWE.

However, this doesn't solve the issue. The problem is a race condition: with equation the associated counter is stepped and then reset at its previous value if \tag is scanned before \end{equation}, but it's too late and hyperref has already provided an anchor.

The tag being the same is not the problem, although I'd be wary of doing it, because it's probably confusing the reader.

Using equation* solves the issue.

\documentclass[12pt]{article}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{mathtools}
\usepackage[hidelinks]{hyperref}

\begin{document}

\section*{Chapter A}
\begin{equation*}\label{star1}\tag{$\star$}
    a^2 + b^2 = c^2
\end{equation*}
The main equation of this chapter is the Pythagorean theorem, \eqref{star1}.

\pagebreak

\section*{Chapter B}
\begin{equation*}\label{star2}\tag{$\star$}
    i^2 = -1
\end{equation*}
The main equation of this chapter is the definition of the imaginary unit, \eqref{star2}.
\end{document}