For anonymous, not-logged-in users, duplicate questions automatically redirect to the dupe target.
In this case, this results in a poor user experience for anonymous users, since they click on the linked question in the sidebar, only to be taken back to where they were. My reaction when I experienced this while not logged in was pretty much "Huh, what's going on here ?".
I suggest that this is fixed, either by:
Disabling the redirect (add
noredirect=1to the URL) for such duplicate questions linked to in the sidebar
Hide duplicate questions which are a duplicate of the question the user is currently looking at from the sidebar.