On the ground, does the northbound carriage of Trafalgar Square butt completely up to the right turn lane into Duncannon Street? If it does, then that would explain why there appears to be two crossings that side of the island. It's one crossing, but it crosses two roads -- and because crossings are marked as nodes in each road, each of those roads will show in map tiles as if they are a separate crossing even though there's no actual gap between them. What was StreetComplete's actual question?