Welcome to the UK List of
TeX Frequently Asked Questions
on the Web

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.