This is the old way of indicating duplicates. It has been changed in 2013; since then the duplicate banner is automatically generated, which has some advantages as outlined in this answer:
Instead of being edited into the question body, the banner is now generated as-needed and displayed separately above the question (in a fashion that appears more or less the same as the edit would've). If the question is reopened, it's removed. As a bonus, this displays the target's answer count, and is automatically updated if the linked target's title is edited. This is also generated for the author as soon as someone votes or flags to close, along with a button to immediately close the question, so the author can know right away that there's an existing solution to be found and close it themselves.
If you look at the timeline of the question, you'll see that it has been closed as an 'exact duplicate' while newer closures have just 'duplicate' instead.
Is there something that users should do to fix questions like these, or should they just leave them as-is?
Users with a dupehammer can reopen the question, remove the banner, and close the question again. This will bump the question, so it should only be done if there's something else that bumped the question already. The old style banner is not optimal but still works, and as the saying goes: if it ain't broke, don't try to fix it.