Why is dueling nowadays illegal?

Sorry if my question sounds crazy or stupid; I just finished watching a movie set in the Middle Ages, when duels of honor were a common practice, so this is what prompted me to ask this question.

If two adults both consent to a duel, then why is it illegal? I'd be really interested in hearing the rationale behind this. Shouldn't people have a right to do as they please with their lives? if people wish to put their life on the line in a duel, it's none of my business. (The fight could take place in a secure place so that other people can't get hurt by one of the combatants.)

Why is dueling nowadays illegal?
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