To repeat an existing equation, one wants not only to have the same mathematics in it, one also wants to re-use the original label it had. The amsmath package comes to our help, here:
\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.
Comprehensive documentation of AMSLaTeX is to be found in amsldoc, in the distribution; it is also available on the web at ftp://ftp.ams.org/pub/tex/doc/amsmath/amsldoc.pdf
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=reuseq