Currently the duplicate links edit check finds duplicates within an entire section. This can be more complicated for the user to work out whether a duplicate should really be removed ("this is identical, but also three long paragraphs away..."), and is also more difficult to compare because it's more likely the links won't be visible at the same time.
We should reduce this so that we warn of duplicates within a single paragraph.
(We could expand to adjacent-paragraphs later, if we want...)