diff --git a/src/Silex/DI/DI.php b/src/Silex/DI/DI.php
index abb1a9b..f4ab35b 100644
--- a/src/Silex/DI/DI.php
+++ b/src/Silex/DI/DI.php
@@ -66,7 +66,8 @@ class DI
     private function getPDO(): PDO
     {
         if ($this->pdo === null) {
-            return new PDO(sprintf('mysql:host=%s;dbname=%s', DB_HOST, DB_DATABASE), DB_USER, DB_PASSWORD);
+            $this->pdo = new PDO(sprintf('mysql:host=%s;dbname=%s', DB_HOST, DB_DATABASE), DB_USER, DB_PASSWORD);
+            $this->pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
         }
         return $this->pdo;
     }
diff --git a/src/Silex/Gateway/UniqueViolation.php b/src/Silex/Gateway/UniqueViolation.php
new file mode 100644
index 0000000..df444d5
--- /dev/null
+++ b/src/Silex/Gateway/UniqueViolation.php
@@ -0,0 +1,15 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Silex\Gateway;
+
+use PDOException;
+
+final class UniqueViolation
+{
+    public static function isUniqueViolation(PDOException $ex): bool
+    {
+        return $ex->errorInfo[1] === 1062; // Mysql
+    }
+}
diff --git a/src/Silex/Gateway/UserGateway.php b/src/Silex/Gateway/UserGateway.php
index 0467f21..34a9b57 100644
--- a/src/Silex/Gateway/UserGateway.php
+++ b/src/Silex/Gateway/UserGateway.php
@@ -5,6 +5,7 @@ declare(strict_types=1);
 namespace Silex\Gateway;
 
 use PDO;
+use PDOException;
 use Silex\Model\User;
 
 class UserGateway
@@ -37,7 +38,14 @@ class UserGateway
     public function insert(User $user): bool
     {
         $req = $this->pdo->prepare('INSERT INTO registered_user (login, password, role) VALUES (:login, :password, :role);');
-        $req->execute(['login' => $user->getLogin(), 'password' => $user->getPasswordHash(), 'role' => $user->getRole()]);
+        try {
+            $req->execute(['login' => $user->getLogin(), 'password' => $user->getPasswordHash(), 'role' => $user->getRole()]);
+        } catch (PDOException $ex) {
+            if (UniqueViolation::isUniqueViolation($ex)) {
+                return false;
+            }
+            throw $ex;
+        }
         $user->setId(intval($this->pdo->lastInsertId()));
         return true;
     }
diff --git a/tables.sql b/tables.sql
index 6f03838..5678201 100644
--- a/tables.sql
+++ b/tables.sql
@@ -1,6 +1,6 @@
 CREATE TABLE registered_user (
     id_user SERIAL PRIMARY KEY,
-    login VARCHAR(32) NOT NULL,
+    login VARCHAR(32) NOT NULL UNIQUE,
     password CHAR(72) NOT NULL, -- BCrypt
     role INT NOT NULL DEFAULT 0
 );
@@ -25,4 +25,4 @@ CREATE TABLE comment (
         ON DELETE CASCADE,
     FOREIGN KEY (author_id) REFERENCES registered_user(id_user)
         ON DELETE CASCADE
-);
\ No newline at end of file
+);