diff --git a/src/test/java/SignUpGUITest.java b/src/test/java/SignUpGUITest.java
index 146ba81..9d4dc38 100644
--- a/src/test/java/SignUpGUITest.java
+++ b/src/test/java/SignUpGUITest.java
@@ -62,13 +62,30 @@ import static org.junit.jupiter.api.Assertions.assertTrue;
 	        assertTrue(isErrorMessageShown(signUpGUI));
 	    }
 	    
+	    @Test
+	    void testExistingUsername() {
+	    SignUpGUI signUpGUI = new SignUpGUI();
+	    signUpGUI.getUsernameField().setText("existinguser");
+	    signUpGUI.getPasswordField().setText("password");
+	    signUpGUI.getConfirmPasswordField().setText("password");
+	    signUpGUI.getBirthdayField().setText("1990-01-01");
+	    signUpGUI.getFirstNameField().setText("John");
+	    signUpGUI.getSurnameField().setText("Doe");
+
+	    signUpGUI.getSignUpButton().doClick(); // Simulate button click
+
+	    // Make sure an error message dialog is shown
+	    assertTrue(isErrorMessageShown(signUpGUI));
+	    }
+	    
 	 // Helper method to check if an error message dialog is shown
 	    private boolean isErrorMessageShown(JFrame frame) {
 	        Component[] components = frame.getComponents();
 	        for (Component component : components) {
 	            if (component instanceof JOptionPane) {
-	                return false;
+	            return false;
 	            }
+	            
 	        }
 	        return true;
 	    }
diff --git a/user.json b/user.json
index de90bc8..d1ad3d8 100644
--- a/user.json
+++ b/user.json
@@ -34,5 +34,14 @@
     "firstName": "John",
     "surname": "Doe",
     "stayLoggedIn": false
+  },
+  {
+    "id": "731fda82-6b51-4f3f-ae97-c9b01c9b4a1a",
+    "userName": "existinguser",
+    "password": "5e884898da28047151d0e56f8dc6292773603d0d6aabbdd62a11ef721d1542d8",
+    "birthday": "1990-01-01",
+    "firstName": "John",
+    "surname": "Doe",
+    "stayLoggedIn": false
   }
 ]
\ No newline at end of file