Remove duplicate cookie headers from the response

[mikejolley]

Submission Review Guidelines:

• I have followed the WooCommerce Contributing Guidelines and the WordPress Coding Standards.
• I have checked to ensure there aren't other open Pull Requests for the same update/change.
• I have reviewed my code for security best practices.
• Following the above guidelines will result in quick merges and clear and detailed feedback when appropriate.

Changes proposed in this Pull Request:

When adding multiple products to the cart programmatically, 2 headers are added per item. This PR ensures that duplicate headers are removed to prevent this occuring.

Closes #39670

How to test the changes in this Pull Request:

Developers:

To test this you can add the following code to your site somewhere. This will add 5 products to your cart when you visit any page, then redirect to the homepage.

add_action(
	'template_redirect',
	function() {
		if ( is_front_page() ) {
			return;
		}

		if ( is_page() ) {
			$products = wc_get_products(
				array(
					'posts_per_page' => 5,
					'status'         => 'publish',
					'type'           => 'simple',
				)
			);
			wc_empty_cart();
			foreach ( $products as $product ) {
				WC()->cart->add_to_cart( $product->get_id(), 1 );
			}
			wp_redirect( get_site_url() );
			exit;
		}
	},
	20
);

After this you can use browser tools to see what headers came back in the response:

1. With the above code in place, go to a page. Wait for the redirect then open network console and view the response of the page with the 302 redirect.
2. With this PR you'll see 2 headers (cart hash and cart items)
3. Without this PR you'll see ~10 headers, with duplicates of the above.

Non-developers:

1. Please smoke test adding items to the cart to test for regressions.
2. Add items to cart from the shop page.
3. Add items to the cart from the product pages.
4. View the cart and ensure results are correct.
5. Test the above as a logged in user, and as a guest.

Changelog entry

• Automatically create a changelog entry from the details below.

Significance

• Patch
• Minor
• Major

Type

• Fix - Fixes an existing bug
• Add - Adds functionality
• Update - Update existing functionality
• Dev - Development related task
• Tweak - A minor adjustment to the codebase
• Performance - Address performance issues
• Enhancement - Improvement to existing functionality

Message

Remove duplicate cookie headers from the response when adding multiple items to the cart programmatically.

Comment

[github-actions]

Test Results Summary

Commit SHA: 3bbb2ed

Test 🧪	Passed ✅	Failed 🚨	Broken 🚧	Skipped ⏭️	Unknown ❔	Total 📊	Duration ⏱️
API Tests	259	0	0	2	0	261	0m 37s
E2E Tests	258	0	0	3	0	261	15m 48s

---

To view the full API test report, click here.
To view the full E2E test report, click here.
To view all test reports, visit the WooCommerce Test Reports Dashboard.

[github-actions]

Hi @opr,

Apart from reviewing the code changes, please make sure to review the testing instructions as well.

You can follow this guide to find out what good testing instructions should look like:
https://github.com/woocommerce/woocommerce/wiki/Writing-high-quality-testing-instructions

[opr]

Working for me! Thanks Mike. Tests not passing though (unrelated) so waiting for a resolution before merging.