skus must be unique, add more variants and skus to products view contain

src/Model/Table/ProductSkusTable.php

@@ -113,6 +113,7 @@ class ProductSkusTable extends Table
     public function buildRules(RulesChecker $rules): RulesChecker
     {
         $rules->add($rules->existsIn(['product_id'], 'Products'), ['errorField' => 'product_id']);
+        $rules->add($rules->isUnique(['sku'], 'SKU must be unique'), ['errorField' => 'sku']);
 
         return $rules;
     }