[Tex/LaTex] Is it ok to load a package twice (as long as no extra options are given)

errorsoption-clashpackages

In

Loading package twice with \RequirePackage causes "option clash"

and

Loading a package more than once

we see that a package can be loaded multiple times, as long as no options or no different options are given.

However, my question is not if this actually works, or if it is implemented in LaTeX this way (well, obviously it is), but if this is a documented behaviour.

This is what I've found in the clsguide about it:

"If a package is always loaded with
\RequirePackage...
or
\usepackage
then,
even if its loading is requested several times, it will be loaded only once."

While this is a clear indication for me that loading a package several times is ok, the author of the catoptions has a different opinion:

"You first loaded xcolor with option cmyk and later without any option. That leads to an option clash that LaTeX can't catch. The catoptions package highlights it."
(Source: catoptions causes option clash with xcolor )

Any more hints from the LaTeX2e documentation regarding this issue?

Example documents which show the problem:

\documentclass{article}
\usepackage{catoptions}
\usepackage[labelfont=bf]{caption}
\usepackage{subcaption}
\begin{document}
Test
\end{document}

\documentclass{article}
\usepackage{catoptions}
\usepackage[balancingshow]{multicol}
\usepackage{doc}
\begin{document}
Test
\end{document}

\documentclass{article}
\usepackage{catoptions}
\usepackage[demo]{graphicx}
\usepackage{rotating}
\begin{document}
Test
\end{document}

All these examples compile fine without using catoptions, but fail when using it.

Best Answer

Experiment 1:

\documentclass{article}
\usepackage[cmyk]{xcolor}
\usepackage{xcolor}
\begin{document}
a
\end{document}

Terminal output:

This is pdfTeX, Version 3.14159265-2.6-1.40.17 (TeX Live 2016) (preloaded format=pdflatex)
 restricted \write18 enabled.
entering extended mode
(./exp.tex
LaTeX2e <2016/03/31> patch level 2
Babel <3.9r> and hyphenation patterns for 83 language(s) loaded.
(/usr/local/texlive/2016/texmf-dist/tex/latex/base/article.cls
Document Class: article 2014/09/29 v1.4h Standard LaTeX document class
(/usr/local/texlive/2016/texmf-dist/tex/latex/base/size10.clo))
(/usr/local/texlive/2016/texmf-dist/tex/latex/xcolor/xcolor.sty
(/usr/local/texlive/2016/texmf-dist/tex/latex/graphics-cfg/color.cfg)
(/usr/local/texlive/2016/texmf-dist/tex/latex/pdftex-def/pdftex.def
(/usr/local/texlive/2016/texmf-dist/tex/generic/oberdiek/infwarerr.sty)
(/usr/local/texlive/2016/texmf-dist/tex/generic/oberdiek/ltxcmds.sty)))
No file exp.aux.
(/usr/local/texlive/2016/texmf-dist/tex/context/base/mkii/supp-pdf.mkii
[Loading MPS to PDF converter (version 2006.09.02).]
) [1{/usr/local/texlive/2016/texmf-var/fonts/map/pdftex/updmap/pdftex.map}]
(./exp.aux) )</usr/local/texlive/2016/texmf-dist/fonts/type1/public/amsfonts/cm
/cmr10.pfb>
Output written on exp.pdf (1 page, 9748 bytes).
Transcript written on exp.log.

Experiment 2:

\documentclass{article}
\usepackage[cmyk]{xcolor}
\usepackage[cmyk]{xcolor}
\begin{document}
a
\end{document}

Same terminal output.

Experiment 3:

\documentclass{article}
\usepackage{xcolor}
\usepackage[cmyk]{xcolor}
\begin{document}
a
\end{document}

Error:

! LaTeX Error: Option clash for package xcolor.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...

Experiment 4:

\documentclass{article}
\usepackage{catoptions}
\usepackage[cmyk]{xcolor}
\usepackage{xcolor}
\begin{document}
a
\end{document}

Error as in experiment 3.

Comments

The catoptions package changes the definition of \@fileswith@ptions, which is internally used by \usepackage (and \RequirePackage).

Without it, the statement in the manual about package loading is true: loading a package with different sets of options raises an error, unless the second (or further) call is with a subset of the original set of options (for example no option) in the first call.

Related Question