I've been reading about famous 18th and 19th century duels and I've always operated under the thought that they were legal - but I see multiple cases (Burr-Hamilton, Jackson-Dickinson, Hickok-Tutt, which is a little late for the time periods I gave) where individuals claimed that the winner should've been, or was outright prosecuted.
With only very minor exception, duelling was illegal in all Euro-American situations from the 16th century onwards. The short answer is that in most cases, especially by the mid-18th century, courts were simply not efficient deterrents, as juries would never convict. I've written several responses about dueling in various localities, with this one being particularly relevant, as well as this comment in the Hamilton AMA.