Equation numbering???

[Angus Leeming]

I have a question about equation numbering.
Suppose I have an equation that I've labelled:

        x^2+y^2=r^2     (eq:circle)

Later in text I want to print it out again, WITH THE SAME
EQ. NUMBER so:

        x^2+y^2=r^2     (eq:circle)

Unfortunately, this does not work. LaTeX gives the second
equation a new number. What to do?

TIA,
Angus