diff --git a/derp/derp_server.go b/derp/derp_server.go
index f85f195a4..30402a332 100644
--- a/derp/derp_server.go
+++ b/derp/derp_server.go
@@ -1275,7 +1275,7 @@ func (s *Server) AddPacketForwarder(dst key.Public, fwd PacketForwarder) {
 			return
 		}
 		if m, ok := prev.(multiForwarder); ok {
-			if _, ok := m[fwd]; !ok {
+			if _, ok := m[fwd]; ok {
 				// Duplicate registration of same forwarder in set; ignore.
 				return
 			}
diff --git a/derp/derp_test.go b/derp/derp_test.go
index d8dfa714b..1aa45363b 100644
--- a/derp/derp_test.go
+++ b/derp/derp_test.go
@@ -712,6 +712,7 @@ func TestForwarderRegistration(t *testing.T) {
 	// Adding a dup for a user.
 	wantCounter(&s.multiForwarderCreated, 0)
 	s.AddPacketForwarder(u1, testFwd(100))
+	s.AddPacketForwarder(u1, testFwd(100)) // dup to trigger dup path
 	want(map[key.Public]PacketForwarder{
 		u1: multiForwarder{
 			testFwd(1):   1,