diff --git a/apps/advanced/common/models/User.php b/apps/advanced/common/models/User.php
index d2c80c0b46..2ec5eb9716 100644
--- a/apps/advanced/common/models/User.php
+++ b/apps/advanced/common/models/User.php
@@ -196,6 +196,7 @@ class User extends ActiveRecord implements IdentityInterface
 
 			['username', 'filter', 'filter' => 'trim'],
 			['username', 'required'],
+			['username', 'unique'],
 			['username', 'string', 'min' => 2, 'max' => 255],
 
 			['email', 'filter', 'filter' => 'trim'],
diff --git a/apps/advanced/frontend/models/SignupForm.php b/apps/advanced/frontend/models/SignupForm.php
index 06b6aa3045..0390424655 100644
--- a/apps/advanced/frontend/models/SignupForm.php
+++ b/apps/advanced/frontend/models/SignupForm.php
@@ -22,6 +22,7 @@ class SignupForm extends Model
 		return [
 			['username', 'filter', 'filter' => 'trim'],
 			['username', 'required'],
+			['username', 'unique', 'targetClass' => '\common\models\User', 'message' => 'This username has already been taken.'],
 			['username', 'string', 'min' => 2, 'max' => 255],
 
 			['email', 'filter', 'filter' => 'trim'],