Warning: There were multiply-defined labels

Question

I got a "LaTeX Warning: There were multiply-defined labels", and I know what it means but my document is pretty big and it has various files. So, how can I find the multiply-defined label?

I am using TeXShop in MacOS.

This is the whole warning (I believe):

------------------------------------------------------------------------------
 Bibliography
------------------------------------------------------------------------------
(./Thesis.bbl [29] [30] [31] [32] [33] [34] [35]) [36] (./Thesis.aux)
LaTeX Warning: There were undefined references.
LaTeX Warning: There were multiply-defined labels.
 )

Answer 1

The log message you quoted is a sort of "summary" of earlier, more detailed warnings, telling you exactly which references are undefined/multiply defined. Scroll up through the log file and you should find more detailed messages that look like the one below:

LaTeX Warning: Label `foo' multiply defined.

Answer 2

There are ways to find duplicate labels in complex documents that have not been mentioned in the other answers. To show how they work I will use a simple (non-complex) test case:

\documentclass{article}

\begin{document}

\section{Section 1}
\label{foo}

\section{Section 2}
\label{foo}

\end{document}

RefTeX

RefTeX can find duplicate labels with the function reftex-find-duplicate-labels which produce a list of all duplicate labels in the document. To use it simply do M-x reftex-find-duplicate-labels. When doing this on the file with the test case it opens a new buffer with the following content:

                MULTIPLE LABELS IN CURRENT DOCUMENT:
 Move point to label and type `r' to run a query-replace on the label
 and its references.  Type `q' to exit this buffer.

 LABEL               FILE
 -------------------------------------------------------------
foo
    ~/test/test.tex
    ~/test/test.tex

Perl

As noted by Walt Mankowski one can use a Perl one-liner to find duplicate labels. The one-liner is as follows:

perl -nE 'say $1 if /(\\label[^}]*})/' *.tex | sort | uniq -c | sort -n

To use it open a terminal and cd to the directory of the document you want to check for duplicate labels in and then execute the one-liner. The output for the test case is as follows:

      2 \label{foo}

---

Relatedly you may also want to check for unused labels.

Answer 3

Besides looking into the .log file, the showlabels package could help you if you would like to check the labels in the output.

To check the input, you could open a terminal and grep for labels:

grep label filename.tex

This gives you a compact overview.

Answer 4

Extension: I just finished a debug tracking down multiply-defined label but only one instance of creating that label. Turns out if you have a table break across pages and a first head is not defined, multiple instances of the label occur.

\begin{longtable}{c}
\caption{my text \label{tab:mine}}\\
Title\\
\endfirsthead
\caption{my text, continued}\\
Title\\
\endhead
blah\\
.
.
.
blah\\
\end{longtable}