This is soo cool, you can create your own coding duel and invite others to compete. It is easily done using the instructions provided by the Pex4Fun site.
Give it a try. First use the template tool, either for F# or C#, then you enter a coding duel name, and then press “Turn this puzzle into a coding duel” This is a well “liked” item, with 2,311 likes currently.
You can also download Pex for Visual Studio.
The interesting thing is that you write a “specification” for the coding duel, so you describe your code.
Other tools that are like this are Small Basic.
The Pex4Fun is powered by community so make sure to dig into the contests and use them for fun!