\documentclass{article}
\usepackage{latexml}
\iflatexml\else
% Dummy definitions for pdflatex
  \def\brackets#1{\{#1\}}
  \def\parens#1{(#1)}
  \def\BoxDup#1{#1#1}
  \def\SaveBox#1{\def\mysavedbox{#1}#1}
  \def\UseBox{\mysavedbox}
\fi
\title{Verify Reuse of Digested Boxes/Whatsits}
\begin{document}
% Duplicated nexted XMDuals
\[ \brackets{\parens{x}} \]
\[ \BoxDup{\brackets{\parens{x}}} \]

% Check reversion of math whatsits
\SaveBox{$\brackets{\parens{x}}$}
\UseBox
\end{document}