Is repeating a mathematical formula or equation in a scientific paper considered bad writing? I'm writing an engineering paper which is filled with equations (physical demonstrations in the theoretical section and numerical methods demonstrations in the methods section).
I want to rewrite one of the first equation I introduced, because I introduced it in a particular context and now I want to use it in another context and show different properties of it. Is this considered a bad practice?
Since all equations are numbered, there would be an equation which is numbered twice. I could solve this problem by not numbering just the second appearance of this equation. Or I could just reference the number, and let the reader go back a few pages. But that would affect the readability of the text (all the other instances in which I reference some previous equation are usually in the same paragraph, so there's no need to turn a lot of pages).
What should I do? Redundancy and readability or conciseness and elegance in spite of readability?
I'd suggest just referencing the previous entry. People reading technical articles are pretty used to skipping back if necessary. It might be confusing to repeat it with a new number, especially.
An alternative would be to repeat it along with the original number, making a note of what you are doing:

...repeating equation (42) x = y + z...

But the reference to the equation is likely enough.