diff --git a/mqttpacket.cpp b/mqttpacket.cpp
index 7387b5d..e4e0283 100644
--- a/mqttpacket.cpp
+++ b/mqttpacket.cpp
@@ -221,6 +221,8 @@ void MqttPacket::handleConnect()
 
         if (sender->getThreadData()->authPlugin.unPwdCheck(username, password) == AuthResult::success)
         {
+            sender->getThreadData()->getSubscriptionStore()->registerClientAndKickExistingOne(sender);
+
             sender->setAuthenticated(true);
             ConnAck connAck(ConnAckReturnCodes::Accepted);
             MqttPacket response(connAck);
diff --git a/subscriptionstore.cpp b/subscriptionstore.cpp
index b8974bc..b0db79f 100644
--- a/subscriptionstore.cpp
+++ b/subscriptionstore.cpp
@@ -51,12 +51,35 @@ void SubscriptionStore::addSubscription(Client_p &client, const std::string &top
         deepestNode->subscribers.push_front(client->getClientId());
     }
 
-    clients_by_id[client->getClientId()] = client;
     lock_guard.unlock();
 
     giveClientRetainedMessages(client, topic);
 }
 
+// Removes an existing client when it already exists [MQTT-3.1.4-2].
+void SubscriptionStore::registerClientAndKickExistingOne(Client_p &client)
+{
+    RWLockGuard lock_guard(&subscriptionsRwlock);
+    lock_guard.wrlock();
+
+    if (client->getClientId().empty())
+        throw ProtocolError("Trying to store client without an ID.");
+
+    std::weak_ptr<Client> existingClient = clients_by_id[client->getClientId()];
+    auto it = clients_by_id.find(client->getClientId());
+
+    if (it != clients_by_id.end() && !it->second.expired())
+    {
+        std::shared_ptr<Client> cl = it->second.lock();
+        logger->logf(LOG_NOTICE, "Disconnecting existing client with id '%s'", cl->getClientId().c_str());
+        cl->setReadyForDisconnect();
+        cl->getThreadData()->removeClient(cl);
+        cl->markAsDisconnecting();
+    }
+
+    clients_by_id[client->getClientId()] = client;
+}
+
 // TODO: should I implement cache, this needs to be changed to returning a list of clients.
 void SubscriptionStore::publishNonRecursively(const MqttPacket &packet, const std::forward_list<std::string> &subscribers) const
 {
diff --git a/subscriptionstore.h b/subscriptionstore.h
index 459e4f6..e2c7a43 100644
--- a/subscriptionstore.h
+++ b/subscriptionstore.h
@@ -12,6 +12,7 @@
 #include "client.h"
 #include "utils.h"
 #include "retainedmessage.h"
+#include "logger.h"
 
 struct RetainedPayload
 {
@@ -44,6 +45,8 @@ class SubscriptionStore
     pthread_rwlock_t retainedMessagesRwlock = PTHREAD_RWLOCK_INITIALIZER;
     std::unordered_set<RetainedMessage> retainedMessages;
 
+    Logger *logger = Logger::getInstance();
+
     void publishNonRecursively(const MqttPacket &packet, const std::forward_list<std::string> &subscribers) const;
     void publishRecursively(std::vector<std::string>::const_iterator cur_subtopic_it, std::vector<std::string>::const_iterator end,
                             std::unique_ptr<SubscriptionNode> &next, const MqttPacket &packet) const;
@@ -51,6 +54,7 @@ public:
     SubscriptionStore();
 
     void addSubscription(Client_p &client, const std::string &topic);
+    void registerClientAndKickExistingOne(Client_p &client);
 
     void queuePacketAtSubscribers(const std::string &topic, const MqttPacket &packet, const Client_p &sender);
     void giveClientRetainedMessages(Client_p &client, const std::string &subscribe_topic);