I believe it is possible (albeit unlikely in the particular example you mention) for there to be two different users with the same username appearing on Usenet search results, since they may have signed up to different news servers. Although you may be searching on a specific news server, the files it has might not necessarily have been uploaded there originally. Remember that news servers continuously share files with each other. However, where the email address is also shown and appears to be valid, I would imagine that this does identify the user precisely.

You are correct to say that the newsreader is verifying fidelity rather than authenticity. In other words, all you know is that your downloaded files are a perfect match for what is stored on the news server. When files are originally uploaded to a news server, they are not verified for authenticity; after all, most of them are probably breaking copyright. Therefore, unless you trust that uploads from a particular user are authentic, I would recommend verifying the overall checksum at your end as well.