Yes, it does affect SEO, assuming the other party duplicated content and URL structure, the OP was not clear on this matter. Just HTML duplication won't (should

Yes, it does affect SEO, assuming the other party duplicated content and URL structure, the OP was not clear on this matter. Just HTML duplication won't (shouldn't) do anything. Google's bots will see 2 pages with similar content and URL structure, and make a judgment call as to which one is canonical, and present that one in search results as the authoritative source. And no, the methods Google uses to determine the canonical source are not known, and in my experience, don't often make much sense to us humans.