diff --git a/broker/src/main/java/io/moquette/broker/Session.java b/broker/src/main/java/io/moquette/broker/Session.java
index 658929378..c227ef8d6 100644
--- a/broker/src/main/java/io/moquette/broker/Session.java
+++ b/broker/src/main/java/io/moquette/broker/Session.java
@@ -377,7 +377,13 @@ private void drainQueueToConnection() {
             int sendPacketId = mqttConnection.nextPacketId();
 
             // Putting it in a map, but the retain is cancelled out by the below release.
-            inflightWindow.put(sendPacketId, msg);
+            EnqueuedMessage old = inflightWindow.put(sendPacketId, msg);
+            if (old != null) {
+                // Something was already there. Release, and free up its slot.
+                old.release();
+                inflightSlots.incrementAndGet();
+            }
+            inflightTimeouts.add(new InFlightPacket(sendPacketId, FLIGHT_BEFORE_RESEND_MS));
             if (msg instanceof SessionRegistry.PubRelMarker) {
                 MqttMessage pubRel = MQTTConnection.pubrel(sendPacketId);
                 mqttConnection.sendIfWritableElseDrop(pubRel);