diff --git a/src/proto/plumtree.rs b/src/proto/plumtree.rs
index 2c114a1f..243cc18a 100644
--- a/src/proto/plumtree.rs
+++ b/src/proto/plumtree.rs
@@ -502,6 +502,7 @@ impl<PI: PeerIdentity> State<PI> {
         // if we already received this message: move peer to lazy set
         // and notify peer about this.
         if self.received_messages.contains_key(&message.id) {
+            debug!("Received duplicate message... ignoring");
             self.add_lazy(sender);
             io.push(OutEvent::SendMessage(sender, Message::Prune));
         // otherwise store the message, emit to application and forward to peers