\documentclass{article}

\def\contents{something}

\DeclareRobustCommand{\defcontents}{\def\contents}

\begin{document}

\def\AAA#1{\bgroup\defcontents{AAA}#1\egroup}
\def\BBB#1{\bgroup\defcontents{BBB}#1\egroup}

\section{\AAA{\contents}}
\section{\BBB{\contents}}
\section{\AAA{\contents}}
\section{\BBB{\contents}}

\end{document}