diff --git a/services/sensorhub-service-mqtt-hivemq/build.gradle b/services/sensorhub-service-mqtt-hivemq/build.gradle
index dc1ce5c82..3cfe9b67f 100644
--- a/services/sensorhub-service-mqtt-hivemq/build.gradle
+++ b/services/sensorhub-service-mqtt-hivemq/build.gradle
@@ -1,6 +1,6 @@
 description = 'MQTT Server (HiveMQ)'
 ext.details = 'Implementation of MQTT client/server API using HiveMQ'
-version = '0.1.0'
+version = '0.1.1'
 
 
 dependencies {
diff --git a/services/sensorhub-service-mqtt-hivemq/src/main/java/org/sensorhub/impl/service/hivemq/OshExtension.java b/services/sensorhub-service-mqtt-hivemq/src/main/java/org/sensorhub/impl/service/hivemq/OshExtension.java
index 83ef6eb3c..429f51e50 100644
--- a/services/sensorhub-service-mqtt-hivemq/src/main/java/org/sensorhub/impl/service/hivemq/OshExtension.java
+++ b/services/sensorhub-service-mqtt-hivemq/src/main/java/org/sensorhub/impl/service/hivemq/OshExtension.java
@@ -17,13 +17,11 @@
 import java.net.InetAddress;
 import java.net.InetSocketAddress;
 import java.nio.ByteBuffer;
-import java.util.Collections;
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.TimeUnit;
-import java.util.concurrent.atomic.AtomicBoolean;
 import org.sensorhub.api.ISensorHub;
 import org.sensorhub.api.comm.mqtt.IMqttServer;
 import org.sensorhub.api.comm.mqtt.MqttException;
@@ -199,19 +197,19 @@ boolean subscribeToTopic(String userId, String clientId, String topic) throws Mq
         var handler = handlers.get(topic);
         if (handler != null)
         {
-            AtomicBoolean notSubscribed = new AtomicBoolean();
-            clientTopics.compute(clientId, (k,v) -> {
-                if (v == null)
-                    v = Collections.newSetFromMap(new ConcurrentHashMap<String, Boolean>());
-                notSubscribed.set(v.add(topic));
-                return v;
-            });
-            
-            if (notSubscribed.get())
+            var topics = clientTopics.computeIfAbsent(clientId, k -> ConcurrentHashMap.newKeySet());
+            if (!topics.contains(topic))
             {
                 log.debug(LOG_SUBSCRIBE_MSG + "Handled by {}",
                     clientId, topic, handler.getClass().getSimpleName());
+
+                // record the topic only after the handler accepts it,
+                // so a retry of a rejected subscribe is checked again
                 handler.onSubscribe(userId, topic, this);
+
+                // a parallel subscribe to the same topic was recorded first
+                if (!topics.add(topic))
+                    handler.onUnsubscribe(userId, topic, this);
             }
             
             return true;
diff --git a/services/sensorhub-service-mqtt-hivemq/src/test/java/org/sensorhub/impl/service/hivemq/TestOshExtension.java b/services/sensorhub-service-mqtt-hivemq/src/test/java/org/sensorhub/impl/service/hivemq/TestOshExtension.java
new file mode 100644
index 000000000..d603807bf
--- /dev/null
+++ b/services/sensorhub-service-mqtt-hivemq/src/test/java/org/sensorhub/impl/service/hivemq/TestOshExtension.java
@@ -0,0 +1,102 @@
+/***************************** BEGIN LICENSE BLOCK ***************************
+
+The contents of this file are subject to the Mozilla Public License, v. 2.0.
+If a copy of the MPL was not distributed with this file, You can obtain one
+at http://mozilla.org/MPL/2.0/.
+
+Software distributed under the License is distributed on an "AS IS" basis,
+WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
+for the specific language governing rights and limitations under the License.
+
+Copyright (C) 2026 GeoRobotix Innovative Research. All Rights Reserved.
+
+******************************* END LICENSE BLOCK ***************************/
+
+package org.sensorhub.impl.service.hivemq;
+
+import static org.junit.Assert.*;
+import java.nio.ByteBuffer;
+import java.security.AccessControlException;
+import org.junit.Test;
+import org.sensorhub.api.comm.mqtt.IMqttServer;
+import org.sensorhub.api.comm.mqtt.IMqttServer.IMqttHandler;
+
+
+public class TestOshExtension
+{
+    static final String TOPIC = "api/systems/1/datastreams/2/observations:data";
+
+    final OshExtension ext = new OshExtension(null, new MqttServer());
+    final CountingHandler handler = new CountingHandler();
+
+
+    public TestOshExtension()
+    {
+        ext.registerHandler("api/", handler);
+    }
+
+
+    @Test
+    public void testRejectedSubscribeIsCheckedOnRetry() throws Exception
+    {
+        handler.allow = false;
+        assertThrows(AccessControlException.class, () -> ext.subscribeToTopic("user", "client1", TOPIC));
+        assertThrows(AccessControlException.class, () -> ext.subscribeToTopic("user", "client1", TOPIC));
+        assertEquals(2, handler.subscribes);
+        assertFalse(ext.clientTopics.get("client1").contains(TOPIC));
+
+        // accepted once the user gets the permission
+        handler.allow = true;
+        assertTrue(ext.subscribeToTopic("user", "client1", TOPIC));
+        assertEquals(3, handler.subscribes);
+        assertTrue(ext.clientTopics.get("client1").contains(TOPIC));
+    }
+
+
+    @Test
+    public void testRepeatedSubscribeCallsHandlerOnce() throws Exception
+    {
+        assertTrue(ext.subscribeToTopic("user", "client1", TOPIC));
+        assertTrue(ext.subscribeToTopic("user", "client1", TOPIC));
+        assertEquals(1, handler.subscribes);
+
+        // other clients are checked separately
+        assertTrue(ext.subscribeToTopic("user", "client2", TOPIC));
+        assertEquals(2, handler.subscribes);
+        assertEquals(0, handler.unsubscribes);
+    }
+
+
+    @Test
+    public void testTopicWithoutHandler() throws Exception
+    {
+        assertFalse(ext.subscribeToTopic("user", "client1", "other/topic"));
+        assertEquals(0, handler.subscribes);
+    }
+
+
+    static class CountingHandler implements IMqttHandler
+    {
+        volatile boolean allow = true;
+        int subscribes, unsubscribes;
+
+        @Override
+        public void onSubscribe(String userID, String topic, IMqttServer server)
+        {
+            subscribes++;
+            if (!allow)
+                throw new AccessControlException("not allowed");
+        }
+
+        @Override
+        public void onUnsubscribe(String userID, String topic, IMqttServer server)
+        {
+            unsubscribes++;
+        }
+
+        @Override
+        public void onPublish(String userID, String topic, ByteBuffer payload, ByteBuffer correlData)
+        {
+        }
+    }
+}