fixed user registration issue

database/migrations/0001_01_01_000000_create_users_table.php

@@ -12,7 +12,7 @@ return new class extends Migration {
     {
         Schema::create('users', function (Blueprint $table) {
             $table->id();
-            $table->string('username');
+            $table->string('username')->unique();
             $table->string('email')->unique();
             $table->timestamp('email_verified_at')->nullable();
             $table->string('password')->nullable();