Something like

private bool IsPlayerOnActiveDuel(string userID)
{
    if (userID == null) return false;
    if (dataDuelists.ContainsKey(userID))
        return true;
    return false;
}

To call it from another plugin like

               if (Duelist != null && Duelist.IsLoaded)
               {
                    if (Duelist.Call<bool>("IsPlayerOnActiveDuel", player.UserIDString))
                        PrintWarning("Player " + player.displayName + " is on a duel right now!");
               }

It's just an example, of course you are free to implement it like you think would be better. The main idea is to have the ability to check if the player is on duel from another plugin.

[HookMethod("inEvent")]
public bool inEvent(BasePlayer player)
{
return InEvent(player);
}

this already exists. does it not work?

Oh, haven't noticed it and didn't found in documentation. Sorry, my bad.

ah didn't realize it wasn't in documentation