Data Migration Tool inserts massive duplicate gallery rows in catalog_product_entity_media_gallery_value causing major slowdown

## Bug Description

After migrating from Magento 1 to Magento 2 using the Data Migration Tool, we identified a severe performance issue in the `Magento\Catalog\Model\Product\Gallery\ReadHandler::sortMediaEntriesByPosition` method. Product image galleries contain massive duplicate entries in the `catalog_product_entity_media_gallery_value` table.

### Detailed Findings

- Example: Product SKU `BC-1-GOLD` (entity_id 3477) only has 3 valid gallery value_ids, but each value_id appears 198 times with the same (value_id, store_id=0, entity_id) tuple in `catalog_product_entity_media_gallery_value`.
- As a result, gallery queries return hundreds of thousands of rows per product (e.g., 198×3 value_ids = 594, joined twice yields 117,612 rows for one product), causing severe slowdowns in catalog operations.
- The Magento 1 database did **not** contain these duplicates — the issue only arose post-migration.

## Root Cause Analysis

- The source repository code at [`src/Migration/Handler/Gallery/InsertValueToEntity.php`](https://github.com/magento/data-migration-tool/blob/06126f33b1972f138aef2464f9e15f193a3df730/src/Migration/Handler/Gallery/InsertValueToEntity.php) is responsible for inserting gallery value-to-entity records. This code does **not** appear to check for pre-existing (value_id, entity_id) records before insertion, potentially allowing for many duplicates during migration.
- Migration config XML only defines document keys, but does not enforce idempotency or uniqueness at the DB/module level for this case.
- There is **no unique constraint** enforced for (value_id, store_id, entity_id) in the target table, increasing the risk.
- No raw direct SQL for inserts is found in this repo, but any repeated/looped execution of the handler or scripts against the same data would amplify duplication.

Magento 2 runs this query

```
SELECT `main`.`value_id`,
`main`.`value` AS `file`,
`main`.`media_type`,
`entity`.`entity_id`,
IFNULL(`value`.`label`, `default_value`.`label`) AS `label`,
IFNULL(`value`.`position`, `default_value`.`position`) AS `position`,
IFNULL(`value`.`disabled`, `default_value`.`disabled`) AS `disabled`,
`default_value`.`label` AS `label_default`,
`default_value`.`position` AS `position_default`,
`default_value`.`disabled` AS `disabled_default`,
IFNULL(`value_video`.`provider`, `default_value_video`.`provider`) AS `video_provider`,
IFNULL(`value_video`.`url`, `default_value_video`.`url`) AS `video_url`,
IFNULL(`value_video`.`title`, `default_value_video`.`title`) AS `video_title`,
IFNULL(`value_video`.`description`, `default_value_video`.`description`) AS `video_description`,
IFNULL(`value_video`.`metadata`, `default_value_video`.`metadata`) AS `video_metadata`,
`default_value_video`.`provider` AS `video_provider_default`,
`default_value_video`.`url` AS `video_url_default`,
`default_value_video`.`title` AS `video_title_default`,
`default_value_video`.`description` AS `video_description_default`,
`default_value_video`.`metadata` AS `video_metadata_default`
FROM `catalog_product_entity_media_gallery` AS `main`
INNER JOIN `catalog_product_entity_media_gallery_value_to_entity` AS `entity`
ON main.value_id = entity.value_id
LEFT JOIN `catalog_product_entity_media_gallery_value` AS `value`
ON main.value_id = value.value_id AND value.store_id = 0 AND value.entity_id = entity.entity_id
LEFT JOIN `catalog_product_entity_media_gallery_value` AS `default_value`
ON main.value_id = default_value.value_id AND default_value.store_id = 0 AND
default_value.entity_id = entity.entity_id
LEFT JOIN `catalog_product_entity_media_gallery_value_video` AS `value_video`
ON value.value_id = value_video.value_id AND value.store_id = value_video.store_id
LEFT JOIN `catalog_product_entity_media_gallery_value_video` AS `default_value_video`
ON default_value.value_id = default_value_video.value_id AND
default_value.store_id = default_value_video.store_id
WHERE (main.attribute_id = '88')
AND (main.disabled = 0)
AND (entity.entity_id = '3477')
ORDER BY IF(value.position IS NULL, default_value.position, value.position) ASC
```

Which in our case yielded 117k results to sort by `usort`

17272 | /s/x/sxara-autokinitou-trion-theseon-bc-1-gold_1.jpg | image | 3477 | Σχάρα αυτοκινήτου για 3 ποδήλατα \| BC-1 GOLD \| example.com | 1 | 0 | Σχάρα αυτοκινήτου για 3 ποδήλατα \| BC-1 GOLD \| example.com | 1 | 0 |   |   |   |   |   |   |   |   |   |  
-- | -- | -- | -- | -- | -- | -- | -- | -- | -- | -- | -- | -- | -- | -- | -- | -- | -- | -- | --
17272 | /s/x/sxara-autokinitou-trion-theseon-bc-1-gold_1.jpg | image | 3477 | Σχάρα αυτοκινήτου για 3 ποδήλατα \| BC-1 GOLD \| example.com | 1 | 0 | Σχάρα αυτοκινήτου για 3 ποδήλατα \| BC-1 GOLD \| example.com | 1 | 0 |   |   |   |   |   |   |   |   |   |  
17272 | /s/x/sxara-autokinitou-trion-theseon-bc-1-gold_1.jpg | image | 3477 | Σχάρα αυτοκινήτου για 3 ποδήλατα \| BC-1 GOLD \| example.com | 1 | 0 | Σχάρα αυτοκινήτου για 3 ποδήλατα \| BC-1 GOLD \| example.com | 1 | 0 |   |   |   |   |   |   |   |   |   |  
17272 | /s/x/sxara-autokinitou-trion-theseon-bc-1-gold_1.jpg | image | 3477 | Σχάρα αυτοκινήτου για 3 ποδήλατα \| BC-1 GOLD \| example.com | 1 | 0 | Σχάρα αυτοκινήτου για 3 ποδήλατα \| BC-1 GOLD \| example.com | 1 | 0 |   |   |   |   |   |   |   |   |   |  
17272 | /s/x/sxara-autokinitou-trion-theseon-bc-1-gold_1.jpg | image | 3477 | Σχάρα αυτοκινήτου για 3 ποδήλατα \| BC-1 GOLD \| example.com | 1 | 0 | Σχάρα αυτοκινήτου για 3 ποδήλατα \| BC-1 GOLD \| example.com | 1 | 0 |   |   |   |   |   |   |   |   |   |  
17272 | /s/x/sxara-autokinitou-trion-theseon-bc-1-gold_1.jpg | image | 3477 | Σχάρα αυτοκινήτου για 3 ποδήλατα \| BC-1 GOLD \| example.com | 1 | 0 | Σχάρα αυτοκινήτου για 3 ποδήλατα \| BC-1 GOLD \| example.com | 1 | 0 |   |   |   |   |   |   |   |   |   |  
17272 | /s/x/sxara-autokinitou-trion-theseon-bc-1-gold_1.jpg | image | 3477 | Σχάρα αυτοκινήτου για 3 ποδήλατα \| BC-1 GOLD \| example.com | 1 | 0 | Σχάρα αυτοκινήτου για 3 ποδήλατα \| BC-1 GOLD \| example.com | 1 | 0 |   |   |   |   |   |   |   |   |   |  
17272 | /s/x/sxara-autokinitou-trion-theseon-bc-1-gold_1.jpg | image | 3477 | Σχάρα αυτοκινήτου για 3 ποδήλατα \| BC-1 GOLD \| example.com | 1 | 0 | Σχάρα αυτοκινήτου για 3 ποδήλατα \| BC-1 GOLD \| example.com | 1 | 0 |   |   |   |   |   |   |   |   |   |  
17272 | /s/x/sxara-autokinitou-trion-theseon-bc-1-gold_1.jpg | image | 3477 | Σχάρα αυτοκινήτου για 3 ποδήλατα \| BC-1 GOLD \| example.com | 1 | 0 | Σχάρα αυτοκινήτου για 3 ποδήλατα \| BC-1 GOLD \| example.com | 1 | 0 |   |   |   |   |   |   |   |   |   |  
17272 | /s/x/sxara-autokinitou-trion-theseon-bc-1-gold_1.jpg | image | 3477 | Σχάρα αυτοκινήτου για 3 ποδήλατα \| BC-1 GOLD \| example.com | 1 | 0 | Σχάρα αυτοκινήτου για 3 ποδήλατα \| BC-1 GOLD \| example.com | 1 | 0 |   |   |   |   |   |   |   |   |   |  

## Proposed Solution

- Add logic in the migration handler and/or resource model to check for existing records before inserting into `catalog_product_entity_media_gallery_value`.

## Additional Information

- No duplicates existed in source (Magento 1) DB before migration.

Start with src/Migration/Handler/Gallery/InsertValueToEntity.php and inspect how gallery value-to-entity records are inserted during migration. Verify the behavior against the catalog_product_entity_media_gallery_value schema and the reported query; done means repeated migration handling does not create duplicate (value_id, store_id, entity_id) rows.

Written by the indexing model from the issue text.