fix #8846: avoid duplicated attribute option values

Author: gomencal

app/code/Magento/Catalog/Model/Product/Attribute/OptionManagement.php

@@ -40,6 +40,17 @@ public function getItems($attributeCode)
      */
     public function add($attributeCode, $option)
     {
+        /** @var \Magento\Eav\Api\Data\AttributeOptionInterface[] $currentOptions */
+        $currentOptions = $this->getItems($attributeCode);
+        if (is_array($currentOptions)) {
+            array_walk($currentOptions, function (&$attributeOption) {
+                /** @var \Magento\Eav\Api\Data\AttributeOptionInterface $attributeOption */
+                $attributeOption = $attributeOption->getLabel();
+            });
+            if (in_array($option->getLabel(), $currentOptions)) {
+                return false;
+            }
+        }
         return $this->eavOptionManagement->add(
             \Magento\Catalog\Api\Data\ProductAttributeInterface::ENTITY_TYPE_CODE,
             $attributeCode,