Why re-uploads won't overwrite images with the upload widget (unsigned uploads)

Unsigned uploads allow users to upload content to your Cloudinary account without the need of signing the requests. Therefore, while we allow unsigned uploads, we protect your account by disallowing overwriting existing ones via unsigned upload requests

For more information:
https://cloudinary.com/documentation/image_upload_api_reference#unsigned_upload_parameters

In case a resource is uploaded, and its public ID is already used by an already existing image, the upload JSON response will include "existing" => "true", which indicates that there's already an uploaded image, and the new upload was rejected. You can check it by:-

if (result[0].existing){ ... }

To support overwriting images, you can use the widget with signed uploads. For more information:
https://cloudinary.com/documentation/upload_widget#signed_uploads