Is repeating a mathematical formula or equation in a scientific paper considered bad writing?
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.