Dueling was never illegal it was the consequences of the duel that could be crimes.

If both men shot guns in a field and no one died or got hurt, there was no crime. Which often happened. As the guns were not accurate.

But if you killed a guy and there were witnesses (both the attendant doctor and priest would turn their back as not to be witnesses) it could be manslaughter or murder.

Industrialisation made it harder to duel due to population density.