\usepackage{amsmath}
...
\begin{equation}
a=b
\label{eq1}
\end{equation}
...
Remember that
\begin{equation}
a=b
\tag{\ref{eq1}}
\end{equation}
Here, the second instance of a=b will be
typeset with a copy, made by the \tag command, of the label of the
first instance.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=reuseq