How to use the combine document class with the siunitx package

Question

I am trying to squish several journal papers together (using the combine document class) to provide the filling for my thesis sandwich. The papers make use of the siunitx package but causes an error.

A similar posting is here: Using the "combine" document class with the listings package from which I have adapted the folowing minimal working example:

\documentclass{combine}                  
\usepackage[load=named]{siunitx}
\begin{document}

\begin{papers}
\coltoctitle{Title}
\coltocauthor{Author}
\label{lbl}
\import{test}
\end{papers}

\end{document}

and in test.tex

\documentclass{article}
\begin{document}
 Hello World
\end{document}

and I get the following error:

ERROR: LaTeX error: "kernel/command-already-defined"    
--- TeX said --- !
! Control sequence \c_siunitx_textsf_tl already defined.

Can anyone help?

Answer 1

As you say, this is caused by code at the beginning of the document. At least for the minimal example, simply bypassing this code in the subsidiary document works:

\documentclass{article}
\makeatletter
\@ifclassloaded{combine}
  {\let\@begindocumenthook\@empty}
  {}
\makeatother
\begin{document}
 Hello World
\end{document}

(\@begindocumenthook is where the \AtBeginDocumentCode is stored.)