Usenet Explorer

An icon in the Newsgroup list for subsrcibed groups.

There is a warning message when you accidently subscribe to a group twice, but you can't cancel the operation.

And this warning message flashes 0.1 second if you subscribe to more than 1 group.

A small icon om the left side of the groupname will be better.

no need, it just won't add duplicate newsgroups by default (there will be option in advanced submenu). it only makes sense regarding regular newsgroups.