Set theorem number and bypass counter

Question

There are times, where I would like to repeat a theorem, for instance, if the proof goes into the appendix. In this case, I would like the two theorems to have the same number, without influencing the counter. Is there any simple way to achieve this?

What I am basically after is this:

\documentclass[a4paper,11pt]{article}
\newtheorem{thm}{Theorem}
\begin{document}
\section{Main}
\begin{thm}
This is theorem 1
\end{thm}
\section{Appendix}
\subsection{Proof of theorem 1}
To prove this theorem we need another theorem:
\begin{theorem}
This is another theorem
\end{theorem}
\setcounter{thm}{0}
\begin{theorem}
This is theorem 1
\begin{proof}
The proof of this theorem follows theorem 2...
\end{proof}
\end{theorem}
\setcounter{thm}{2}
\end{document}

But without the hassle of manually having to keep track of how far the counter has come.

Answer 1

How's this? I define an environment {repeatthm} which takes a mandatory argument which should be the label of the theorem being repeated. The repeat theorem labels itself Theorem \ref{#1}. (So if the reference is undefined it will appear as Theorem ??.)

\documentclass{article}
\usepackage{amsthm}
\newtheorem{thm}{Theorem}

\makeatletter
\newtheorem{repeatthm@}{Theorem}
\newenvironment{repeatthm}[1]{%
    \def\therepeatthm@{\ref{#1}}
    \repeatthm@
}
{\endrepeatthm@}
\makeatother

\begin{document}
\section{Main}
\begin{thm}
This is thm 1
\end{thm}

\begin{thm}
\label{a-thm}
This is theorem 2
\end{thm}
\section{Appendix}
\subsection{Proof of thm 1}
To prove this theorem we need another theorem:
\begin{thm}
This is another theorem.
\end{thm}

\begin{repeatthm}{a-thm}
This is theorem 2 again.
\end{repeatthm}
\begin{proof}
The proof of this theorem follows thm 2...
\end{proof}

\begin{repeatthm}{unknown-thm}
This is a repeated theorem with an incorrect label.
\end{repeatthm}
\end{document}