Picture "sources" downcast converter duplicates "picture" element when "sources" model attribute updated

[f1ames]

📝 Provide detailed reproduction steps (if any)

1. Go to CKBox manual test (e.g. http://localhost:8125/ckeditor5-ckbox/tests/manual/ckbox.html).
2. Insert any image from CKBox as a first editor element (it's important for image to have picture element).
3. Run editor.model.change( writer => writer.setAttribute('sources', [ { srcset: './sample.png' } ], editor.model.document.getRoot().getChild(0)) );.
4. Take a look on image element in CKE5 inspector in View tab.

✔️ Expected result

The view data has structure like figure > picture > sources + img.

❌ Actual result

The view data has structure like figure > picture > sources > picture > sources + img.

❓ Possible solution

The sources downcast converter does not check the structure of the updated element and always inserts new picture element. The good news is that on editor.getData() resulting HTML is correct without nested strucure.

📃 Other details

• Browser: Chrome
• OS: macOS
• First affected CKEditor version: Not sure, tested on latest one (43.1.1).
• Installed CKEditor plugins: See mentioned manual test.

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