How can we prevent that from happening?
This happens for all old duplicate closures, look e.g. here for more examples. Questions closed after February 4th, 2013 always show a link to the so-called 'original' in their banner.
Back then, the duplicate banner was edited into the post itself:

You can start to edit the post to see the 'source code' of the banner. There is no 'official' link from this question to the original, hence the blue banner stays empty.
The only way to solve this is to reopen the question, and close it again as a duplicate (and edit out the original banner). We do this sometimes on Meta Stack Exchange (there are many people with dupehammers there); on main sites, basically only ♦ moderators can do that. But as you can see, they would have to do that on many posts. I don't think that's an option.