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.