Option clash for package

The error message

! LaTeX Error: Option clash for package footmisc

means what it says — your document contains a (potentially) clashing pair of options; sadly, it is not always obvious how the error has arisen.

If you simply write:

\usepackage[a]{foo}
...
\usepackage{foo}

LaTeX is happy, as it is with:

\usepackage[a]{foo}
...
\usepackage[a]{foo}

since LaTeX can see there’s no conflict (in fact, the second load does nothing).

Similarly,

\usepackage[a,b]{foo}
...
\usepackage[a]{foo}

produces no error and does nothing for the second load.

However

\usepackage[a]{foo}
...
\usepackage[b]{foo}

produces the error; even if option ‘b’ is an alias for option ‘a’ — LaTeX doesn’t “look inside” the package to check anything like that.

The general rule is: the first load of a package defines a set of options; if a further \usepackage or \RequirePackage also calls for the package, the options on that call may not extend the set on the first load.

Fortunately, the error (in that sort of case) is easily curable once you’ve examined the preamble of your document.

Now, suppose package foo loads bar with option b, and your document says:

\usepackage{foo}
...
\usepackage[a]{bar}

\usepackage[a]{bar}
...
\usepackage{foo}

the error will be detected, even though you have only explicitly loaded bar once. Debugging such errors is tricky: it may involve reading the logs (to spot which packages were called), or the documentation of package foo.

This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=optclash