Is there any place in the world where it is still legally possible to kill someone in a duel and not be charged? If not, when and where was the last place to outlaw the practice?
According to Wikipedia, duelling was legal in Uruguay from 1921 to 1971. It cites this article and I have found in the source that the law was passed in 1920, although I haven't found that it ended in 1971. However, several politically motivated duels were fought in Uruguay in 1971, and one newspaper claimed at least one to be legal.
There are many parts of the world that are still tribal and governed by tribal laws. (e.g., Many parts of Central America, South America, Africa, Afghanistan, outside of Kabul, etc.)
I would imagine dueling is probably legal in one or more of those places.