Someone has added a really useful section to the Overpass API by Example wiki page, about detecting highways tagged with sidewalk tag, that have separately mapped sidewalk nearby. See this example Overpass Turbo link, which amongst others highlights this road that someone has subsequently added separate sidewalks to, but not changed the road to sidewalk=separate.
That’s actually a pretty common thing to do (I’ve certainly done it plenty of times), so it’s really useful to have something to detect them.
It’s not quite the finished article, because it only looks for e.g. ["highway"]["sidewalk"] and not ["sidewalk:left"] et al, and I can think of places where there’s both an immediate roadside sidewalk and a separately mapped footwalk=sidewalk only a couple of meters away, but it’s a really useful tool to use to start looking for potential issues.
