Do 'multiply defined labels' refer to the last labelled item?

Question

In my work as LaTeX typesetter I often deal with multiply defined labels warnings. Most of them are about equations.

My question is, do the \ref command "always" refer to the last labelled item? In my experience it is so, but I'd like to know if there are cases (e.g. settings o packages) in which this is not true.

E.g. if I have:

\documentclass[11pt]{article}

\begin{document}

\section{Introduction}

Equation~(\ref{eq1})
\begin{equation}
1 +1 =3
\label{eq1}
\end{equation}
Equation~(\ref{eq1})
\begin{equation}
2 +2 =5
\label{eq1}
\end{equation}

\end{document}

both refs refers to the last equation.

Note. We always ask the author(s) to check about the warning but my company policy is to remove the warning "before" sending the proofs. I'm searching for a "scripting" way to manage these cases, commenting all the multiply defined labels except for the last one, yet I need to know if this is a safe approach.

Answer 1

If you look at the .aux file, then you see

\relax 
\@writefile{toc}{\contentsline {section}{\numberline {1}Introduction}{1}}
\newlabel{eq1}{{1}{1}}
\newlabel{eq1}{{2}{1}}

As part of \begin{document}, LaTeX reads the .aux file (as produced in the previous run) and the command \newlabel{eq1}{{1}{1}} essentially does

\global\@namedef{r@eq1}{{1}{1}}

which is then overridden by the later

\global\@namedef{r@eq1}{{2}{1}}

with the effect that \ref{eq1} will point to equation 2, because \ref{eq1} checks for the macro \r@eq1 (produced with \csname, so numbers are legal in it) to be defined and, in case, it uses the contents in the first brace pair from its replacement text (\pageref uses the second).

When \end{document} is processed, the .aux file produced in the current run is read in, but \newlabel does different things, among which checking whether the label has already appeared.

The solution is very simple: never duplicate labels. In case there are, you need to ask the author to fix them. Using the last one may not be the author’s intention.