Add: Prevent multiple logins by checking for existing user session

src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/login/LoginHandler.java

@@ -4,20 +4,34 @@ import ch.unibas.dmi.dbis.cs108.casono.server.domain.user.User;
 import ch.unibas.dmi.dbis.cs108.casono.server.domain.user.UserFactory;
 import ch.unibas.dmi.dbis.cs108.casono.server.domain.user.UserRegistry;
 import ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.CommandHandler;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.ErrorResponse;
 import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.dispatcher.ResponseDispatcher;
+import java.util.Optional;
 
 public class LoginHandler implements CommandHandler<LoginRequest> {
     private final ResponseDispatcher responseDispatcher;
+    private final UserRegistry userRegistry;
     private final UserFactory userFactory;
 
     public LoginHandler(ResponseDispatcher responseDispatcher, UserRegistry userRegistry) {
         this.responseDispatcher = responseDispatcher;
+        this.userRegistry = userRegistry;
         this.userFactory = new UserFactory(userRegistry);
     }
 
     @Override
     public void execute(LoginRequest request) {
-        User user = userFactory.create(request.getUsername(), request.getContext().sessionId());
+        Optional<User> existingUser = userRegistry.getBySessionId(request.getSessionId());
+        if (existingUser.isPresent()) {
+            responseDispatcher.dispatch(
+                    new ErrorResponse(
+                            request.getContext(),
+                            "ALREADY_LOGGED_IN",
+                            "This session is already associated with an active user."));
+            return;
+        }
+
+        User user = userFactory.create(request.getUsername(), request.getSessionId());
         responseDispatcher.dispatch(
                 new LoginResponse(request.getContext(), user.getName(), user.getId()));
     }