# Give two equations the same label but different hyperref anchors

In the introduction of a chapter, I define an equation. Several pages later, I then recall that equation before proceeding to do things with it. I want to assign it the same label in both instances, because it is the same equation, and I know that this can be achieved using \tag{\ref{eq:first-label}} (see MWE below). My problem is this: I would still like to be able to provide specific links to the two different instances of the equation: when referencing that equation in the introduction, I of course want to link to the first instance of the equation. Later in the chapter, I do not want the link to go to the first instance in the introduction, but to the second instance (which is much more recent in the text).

It took me a bit of tinkering to figure this out, so I thought I'd share my solution in case it's useful for someone else. Here's a MWE that does what I want, using the \hypertarget and \hyperlink commands provided by hyperref.

\documentclass{article}

\usepackage{amsmath}
\usepackage{lipsum}
\usepackage{hyperref}

\begin{document}

$$a+b=c \label{eq:first-instance}$$

\lipsum[1-3]

This should link to the first instance of the equation: \eqref{eq:first-instance}.

$$\hypertarget{eq:second-instance}{a+b=c} \tag{\ref{eq:first-instance}}$$