fix magento#8846: avoid duplicated attribute option values

Author: gomencal

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

@@ -40,11 +40,19 @@ public function getItems($attributeCode)
      */
     public function add($attributeCode, $option)
     {
-        return $this->eavOptionManagement->add(
-            \Magento\Catalog\Api\Data\ProductAttributeInterface::ENTITY_TYPE_CODE,
-            $attributeCode,
-            $option
-        );
+        $currentOptions = $this->getItems($attributeCode);
+        array_walk($currentOptions, function (&$attributeOption) {
+            /** @var \Magento\Eav\Api\Data\AttributeOptionInterface $attributeOption */
+            $attributeOption = $attributeOption->getLabel();
+        });
+        if (!in_array($option->getLabel(), $currentOptions)) {
+            return $this->eavOptionManagement->add(
+                \Magento\Catalog\Api\Data\ProductAttributeInterface::ENTITY_TYPE_CODE,
+                $attributeCode,
+                $option
+            );
+        }
+        return false;
     }
 
     /**