[PluginReference] Plugin ArenaTournament;

private bool IsOnTournament(ulong userid)
{
  return ArenaTournament != null && ArenaTournament.Call<bool>("IsOnTournament", userid);
}

private bool IsDueling(BasePlayer player)
{
  return ArenaTournament != null && ArenaTournament.Call<bool>("IsOnTournament", player);
}