diff --git a/app/src/main/java/lightcontainer/domains/client/Context.java b/app/src/main/java/lightcontainer/domains/client/Context.java
index 8e63323..5ff19c4 100644
--- a/app/src/main/java/lightcontainer/domains/client/Context.java
+++ b/app/src/main/java/lightcontainer/domains/client/Context.java
@@ -1,5 +1,7 @@
 package lightcontainer.domains.client;
 
+import lightcontainer.repository.ClientHandlerRepository;
+import lightcontainer.repository.ReadOnlyClientRepository;
 import lightcontainer.storage.File;
 import lightcontainer.storage.ReadOnlyFile;
 import lightcontainer.storage.Repository;
@@ -18,6 +20,7 @@ public class Context {
     // Variables
     private final Repository repository;
     private RequestBundle requestBundle;
+    private ReadOnlyClientRepository clientRepository;
 
     /**
      * Login de l'utilisateur
@@ -30,8 +33,9 @@ public class Context {
     private String domain;
 
     // Constructeur
-    public Context(Repository repository) {
+    public Context(Repository repository, ReadOnlyClientRepository clientRepository) {
         this.repository = repository;
+        this.clientRepository = clientRepository;
     }
 
 
@@ -43,6 +47,10 @@ public class Context {
      * @return TRUE si l'utilisateur a pu être créé
      */
     public boolean createUser(String login, String password) {
+        if (clientRepository.hasClient(login)) { // Empêcher double connection
+            return false;
+        }
+
         try {
             String key = AES_GCM.generateSecretKey();
             String hashedPassword = BCryptHasher.hashPassword(password);
@@ -73,7 +81,12 @@ public class Context {
      * @return TRUE si l'utilisateur a été authentifié
      */
     public boolean signIn(String login, String password) {
-        if (this.repository.verifyUser(login, password)) {
+        if (login == null || password == null) {
+            return false;
+        }
+
+        // Empêcher double connection
+        if (!clientRepository.hasClient(login) && this.repository.verifyUser(login, password)) {
             this.login = login;
             return true;
         }
diff --git a/app/src/main/java/lightcontainer/domains/server/UnicastServerListener.java b/app/src/main/java/lightcontainer/domains/server/UnicastServerListener.java
index 5603d5f..cc704e2 100644
--- a/app/src/main/java/lightcontainer/domains/server/UnicastServerListener.java
+++ b/app/src/main/java/lightcontainer/domains/server/UnicastServerListener.java
@@ -55,7 +55,7 @@ public class UnicastServerListener implements Runnable {
                 SSLSocket client = (SSLSocket) this.server.accept();
 
                 // Create a new Handler client by passing these dependencies to it
-                ClientHandler clientHandler = new ClientHandler(this.repository, client, ffe, protocolRep, new Context(repositoryStorage));
+                ClientHandler clientHandler = new ClientHandler(this.repository, client, ffe, protocolRep, new Context(repositoryStorage, repository));
                 // Add the client handler to its repository (clienthandlerrepository)
                 this.repository.addClient(clientHandler);
                 // Start the thread
diff --git a/app/src/main/java/lightcontainer/interfaces/UnicastCHR.java b/app/src/main/java/lightcontainer/interfaces/UnicastCHR.java
index 21441b3..51f54ef 100644
--- a/app/src/main/java/lightcontainer/interfaces/UnicastCHR.java
+++ b/app/src/main/java/lightcontainer/interfaces/UnicastCHR.java
@@ -5,11 +5,12 @@ import lightcontainer.domains.server.MulticastServerListener;
 import lightcontainer.domains.server.UnicastServerListener;
 import lightcontainer.protocol.ProtocolWriter;
 import lightcontainer.repository.ClientHandlerRepository;
+import lightcontainer.repository.ReadOnlyClientRepository;
 
 /**
  * A communication interface between a {@link ClientHandler} and the {@link ClientHandlerRepository}.
  */
-public interface UnicastCHR {
+public interface UnicastCHR extends ReadOnlyClientRepository {
     /**
      * Setter, allow to define the ServerListener of a repository.
      *
@@ -24,6 +25,7 @@ public interface UnicastCHR {
      */
     void addClient(ClientHandler client);
 
+
     /**
      * Permet de demander la déconnection d'un client
      *
diff --git a/app/src/main/java/lightcontainer/repository/ClientHandlerRepository.java b/app/src/main/java/lightcontainer/repository/ClientHandlerRepository.java
index 9c69784..b1f1731 100644
--- a/app/src/main/java/lightcontainer/repository/ClientHandlerRepository.java
+++ b/app/src/main/java/lightcontainer/repository/ClientHandlerRepository.java
@@ -55,6 +55,22 @@ public class ClientHandlerRepository implements AutoCloseable, UnicastCHR {
         this.handlers.add(client);
     }
 
+    /**
+     * Permet de savoir si un client est déjà connecté avec ce login.
+     *
+     * @param login Login du client.
+     */
+    @Override
+    public boolean hasClient(String login) {
+        for (ClientHandler client : handlers) {
+            if (client.getLogin() != null && client.getLogin().equals(login)) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+
     @Override
     public void disconnect(ClientHandler client) {
         if (handlers.remove(client))
diff --git a/app/src/main/java/lightcontainer/repository/ReadOnlyClientRepository.java b/app/src/main/java/lightcontainer/repository/ReadOnlyClientRepository.java
new file mode 100644
index 0000000..6244a2c
--- /dev/null
+++ b/app/src/main/java/lightcontainer/repository/ReadOnlyClientRepository.java
@@ -0,0 +1,12 @@
+package lightcontainer.repository;
+
+public interface ReadOnlyClientRepository {
+
+    /**
+     * Permet de savoir si un client est déjà connecté avec ce login.
+     *
+     * @param login Login du client.
+     */
+    boolean hasClient(String login);
+
+}