diff --git a/pubsub.go b/pubsub.go
index a9fe375..a3638ee 100644
--- a/pubsub.go
+++ b/pubsub.go
@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"math/rand"
 	"runtime"
+	"sync"
 	"sync/atomic"
 	"time"
 
@@ -106,8 +107,10 @@ type PubSub struct {
 	blacklist     Blacklist
 	blacklistPeer chan peer.ID
 
-	peers        map[peer.ID]chan *RPC
-	seenMessages *timecache.TimeCache
+	peers map[peer.ID]chan *RPC
+
+	seenMessagesMx sync.Mutex
+	seenMessages   *timecache.TimeCache
 
 	// key for signing messages; nil when signing is disabled (default for now)
 	signKey crypto.PrivKey
@@ -552,12 +555,22 @@ func (p *PubSub) notifySubs(msg *pb.Message) {
 
 // seenMessage returns whether we already saw this message before
 func (p *PubSub) seenMessage(id string) bool {
+	p.seenMessagesMx.Lock()
+	defer p.seenMessagesMx.Unlock()
 	return p.seenMessages.Has(id)
 }
 
 // markSeen marks a message as seen such that seenMessage returns `true' for the given id
-func (p *PubSub) markSeen(id string) {
+// returns true if the message was freshly marked
+func (p *PubSub) markSeen(id string) bool {
+	p.seenMessagesMx.Lock()
+	defer p.seenMessagesMx.Unlock()
+	if p.seenMessages.Has(id) {
+		return false
+	}
+
 	p.seenMessages.Add(id)
+	return true
 }
 
 // subscribedToMessage returns whether we are subscribed to one of the topics
@@ -649,7 +662,9 @@ func (p *PubSub) pushMsg(vals []*topicVal, src peer.ID, msg *Message) {
 		return
 	}
 
-	p.publishMessage(src, msg.Message)
+	if p.markSeen(id) {
+		p.publishMessage(src, msg.Message)
+	}
 }
 
 func (p *PubSub) validateWorker() {
@@ -674,6 +689,13 @@ func (p *PubSub) validate(vals []*topicVal, src peer.ID, msg *Message) {
 		}
 	}
 
+	// we can mark the message as seen now that we have verified the signature
+	// and avoid invoking user validators more than once
+	id := msgID(msg.Message)
+	if !p.markSeen(id) {
+		return
+	}
+
 	if len(vals) > 0 {
 		select {
 		case p.validateThrottle <- struct{}{}:
@@ -779,12 +801,6 @@ func (p *PubSub) validateSingleTopic(val *topicVal, src peer.ID, msg *Message) b
 }
 
 func (p *PubSub) publishMessage(from peer.ID, pmsg *pb.Message) {
-	id := msgID(pmsg)
-	if p.seenMessage(id) {
-		return
-	}
-	p.markSeen(id)
-
 	p.notifySubs(pmsg)
 	p.rt.Publish(from, pmsg)
 }