Fix error visiting compare page without items

[indykoning]

Description (*)

Going incognito and visiting /catalog/product_compare/index/ will cause an

Item (Magento\Catalog\Model\Product\Interceptor) with the same ID "xxx" already exists.

Error on many live webshops. This is because the database table looks somewhat like this

catalog_compare_item_id	visitor_id	customer_id	product_id	store_id	list_id
23159	4690502		19028	1
22917	0		18906	1	711
23034	0		18868	1	732
22916	0		18851	1	711
21991	0		18783	1	537
21573	0		18747	1	451
22196	0		18740	1	585
22485	0		18706	1	644
22111	0		18705	1	568
22259	0		18689	1	596
22133	0		18689	1	573
21802	0		18689	1	492

Thus if we only get items with visitor_id 0 and customer_id NULL we will get duplicate products (and compare product from other customers...)

Manual testing scenarios (*)

1. Ensure your catalog_compare_items somewhat matches the example above (duplicate product id's on different lists)
2. Visit the /catalog/product_compare/index/ route in incognito, so no visitor id, customer id, or list id is assigned to you
3. See the error that occurs.

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)
• README.md files for modified modules are updated and included in the pull request if any README.md predefined sections require an update
• All automated tests passed successfully (all builds are green)

Resolved issues:

1. resolves [Issue] Fix error visiting compare page without items #40205: Fix error visiting compare page without items

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[engcom-Hotel]

@magento create issue