Re-using an equation

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:

snippet.latex

\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.