Redirect the tag to the particular article that it currently references. Only show users the "correct" url that you want them to use in the future (history, bookmarks, sharing, etc).
When there are two articles with the tag, they will both be shown on the 'publishing' page, in their entirety, newest first. See [1] for an example.
Redirecting would be akin to Google redirecting you to the current top result (like I'm feeling lucky, but for all searches)
Anyway, I didn't mean to argue. Have a nice evening :)