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:

Here, the second instance of a=b will be typeset with a copy, made by the\usepackage{amsmath} ... \begin{equation} a=b \label{eq1} \end{equation} ... Remember that \begin{equation} a=b \tag{\ref{eq1}} \end{equation}

`\tag`

command, of the label of the
first instance.
- amsmath.sty
- Distributed as part of AMSLaTeX amslatex

This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=reuseq