message: Do not add vertex if it's already in validator's state.
diff: @@ -168,6 +168,7 @@ impl<C: Context> Highway<C> {
  &mut self,
  ValidVertex(vertex): ValidVertex<C>,
  ) -> Vec<Effect<C>> {
  + if !self.has_vertex(&vertex) {
  match vertex {
  Vertex::Vote(vote) => self.add_valid_vote(vote),
  Vertex::Evidence(evidence) => {
  @@ -175,6 +176,9 @@ impl<C: Context> Highway<C> {
  vec![]
  }
  }
  + } else {
  + vec![]
  + }
  }
  /// Returns whether the vertex is already part of this protocol state.
  