magento-engcom/import-export-improvements#103 Fail product import validation when multiselect columns contain duplicate values

[pogster]

Description

Introduced a validation failure with message on product import attribute validation of multiselect columns that contain the same value multiple times.

Without this, in 2.2.2 there were reportedly errors on the indexing run. On 2.3, the indexer errors could not be reproduced, but the values are still saved to catalog_product_entity_varchar multiple times.

@dmanners and I agreed that a validation failure in such a case would be appropriate, forcing the user to supply us with clean data.

Fixed Issues

1. Duplicate multiselect values result in Product EAV indexing errors #103

Manual testing scenarios

1. Create a multiselect attribute, fill it with values (e.g. Dutch, Englisch, French, Chinese)
2. Create an import sheet with a column for the multiselect attribute
3. Fill in duplicate values, for example Dutch|English|French|Chinese|English.
4. Import the sheet.

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)