Fixed issue related to 'product_attribute' attribute. Added condition for validating product attribute.

Author: GovindaSharma

app/code/Magento/Catalog/Controller/Adminhtml/Product/Attribute/Validate.php

@@ -100,7 +100,7 @@ public function execute()
             $attributeCode
         );
 
-        if ($attribute->getId() && !$attributeId) {
+        if ($attribute->getId() && !$attributeId || $attributeCode === 'product_type') {
             $message = strlen($this->getRequest()->getParam('attribute_code'))
                 ? __('An attribute with this code already exists.')
                 : __('An attribute with the same code (%1) already exists.', $attributeCode);