fix #8846: avoid duplicated attribute option values

[gomencal]

… #8846

Description

When adding an attribute option, check if its label exists for current attribute. If so, doesn't add the option and return false.

Fixed Issues

1. Attribute option value uniqueness is not checked if created via REST Api #8846 Attribute option value uniqueness is not checked if created via REST Api

Manual testing scenarios

Same testing described at issues #8846. Add an attribute option via API twice.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[orlangur]

What is current behavior for identical labels added via Admin UI?

[gomencal]

What is current behavior for identical labels added via Admin UI?

HI @orlangur, it's validated via js, in app/code/Magento/Catalog/view/adminhtml/templates/catalog/product/attribute/options.phtml:
<script type="text/x-magento-init"> { "*": { "Magento_Catalog/js/options": { "attributesData": <?= /* @noEscape */ json_encode($values, JSON_HEX_QUOT) ?>, "isSortable": <?= (int)(!$block->getReadOnly() && !$block->canManageOptionDefaultOnly()) ?>, "isReadOnly": <?= (int)$block->getReadOnly() ?> }, "Magento_Catalog/catalog/product/attribute/unique-validate": { "element": "required-dropdown-attribute-unique", "message": "<?= $block->escapeHtml(__("The value of Admin must be unique.")) ?>" } } } </script>

[orlangur]

Oh :O Didn't expect we still have some client side validations stricter than server side.