I'm trying to create a theorem environment that I can use to repeat a theorem that has been previously stated in a book but number it the same as the 1st instance. I found some sort of solution:
\documentclass{article}
\usepackage{amsthm}
\newtheorem{theorem}{Theorem}
\newcounter{thmC}
\newtheorem{rethm}[thmC]{Theorem}
\newenvironment{retheorem}[1]%
{\setcounter{thmC}{#1}%
\addtocounter{thmC}{-1}%
\begin{rethm}}%
{\end{rethm}\ignorespacesafterend}
\begin{document}
\begin{theorem}\label{theorem:Hi}
Hi this is my theorem.
\end{theorem}
\begin{theorem}\label{theorem:another}
Another theorem.
\end{theorem}
\begin{theorem}\label{theorem:third}
third theorem
\end{theorem}
\begin{retheorem}{\ref{theorem:third}}
third theorem
\end{retheorem}
\begin{retheorem}{\ref{theorem:another}}
Another theorem.
\end{retheorem}
\begin{retheorem}{\ref{theorem:Hi}}
Hi this is my theorem.
\end{retheorem}
\end{document}
It nearly works but there is a spacing issue.
My further concern is will this approach work for theorems labeled with the section
\numberwithin{theoremC}{section}
\newtheorem{theorem}[theoremC]{Theorem}