When I use \widecheck twice, it erroneously double prints the entry in the first \widecheck on top of the entry in the second \widecheck. Here is a minimal working example:
\documentclass{article}
\usepackage{amsmath}
\usepackage{mathabx}
\begin{document}
[
A = \widecheck{\widehat{A}}
= \sqrt{2\pi} \widecheck{(\hat\varphi\hat\varphi)}
= \varphi* \varphi
]
\end{document}
The output looks like:
\compositeaccentsworkaround I proposed works also in this case. – egreg Jul 22 '22 at 17:22