diff --git a/roomserver/internal/input/input.go b/roomserver/internal/input/input.go
index 27809e54..7d58b80b 100644
--- a/roomserver/internal/input/input.go
+++ b/roomserver/internal/input/input.go
@@ -33,6 +33,7 @@ import (
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/nats-io/nats.go"
 	"github.com/prometheus/client_golang/prometheus"
+	"github.com/sirupsen/logrus"
 	log "github.com/sirupsen/logrus"
 	"github.com/tidwall/gjson"
 )
@@ -66,6 +67,8 @@ func (r *Inputer) workerForRoom(roomID string) *phony.Inbox {
 	return inbox.(*phony.Inbox)
 }
 
+var eventIDsQueued sync.Map
+
 // onMessage is called when a new event arrives in the roomserver input stream.
 func (r *Inputer) Start() error {
 	_, err := r.JetStream.Subscribe(
@@ -80,6 +83,16 @@ func (r *Inputer) Start() error {
 				_ = msg.Term()
 				return
 			}
+
+			eventID := inputRoomEvent.Event.EventID()
+			_, existed := eventIDsQueued.LoadOrStore(eventID, struct{}{})
+			if existed {
+				logrus.Errorf("XXX: We already received event ID %s", eventID)
+				_ = msg.Term()
+				return
+			}
+			defer eventIDsQueued.Delete(eventID)
+
 			roomserverInputBackpressure.With(prometheus.Labels{"room_id": roomID}).Inc()
 			worker := r.workerForRoom(roomID)
 			worker.Act(nil, func() {