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As you are not provided any MWE, I assumed that you are using the standard supporting files, and the MWE is:
\documentclass{article}
\usepackage{amsmath,amsthm}
\usepackage{lipsum}
\begin{document}
\newtheorem{theorem}{Theorem}
\numberwithin{theorem}{subsection}
\section{Section}
\subsection{Subsection}
\begin{theorem}
\lipsum[...
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