Frequently Asked Question List for TeX

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.