I was surprised the site did not have an icon, so I searched for duck and found the duckduckgo icon. are you sure it's opensource ?
Their inclusion probably isn't an issue in itself, but the blanket guidance in the copy above the result will be.
> The icons below can be used for Personal & Commercial purposes. They can be used on your website, branding and designs.
Obviously assets relating to existing brands will never be permitted for use in the branding of an unaffiliated party. onassar may want to update the guidelines to note such exclusions.
> Naturally, trademarks apply to any brand icons or logos above. That means that you can use them in practice (for example, linking to the respective service), but that the company likely has limitations on how else they can be used.
I need to do some work to communicate that better :)