Show duplicated nodes via Overpass

As Overpass API is not the most common tool for this task, I will also mention a few more mainstream alternatives.

Option 1: Overpass API

This query will first determine all nodes in the current bounding box. For each node found, we will then start looking for some nodes at exactly the same location. The result will return all pairs of duplicate nodes found in the current bbox.

[bbox:{{bbox}}];
node;
foreach->.a(
    node(around.a:0)->.b;
    (.b; - .a);
    out meta;
);

Unfortunately, this approach is extremely slow with out meta; (NB: this seems to be a bug, I've created a Github issue for this). If you don't need metadata information, just replace out meta; by out; and you will get the response almost instantly.

Try it on overpass turbo: very slow (with metadata) and fast (without metadata)

Option 2:

I'd really recommend to also look at keep right! (analysis data is a few weeks old): see this example for a layer with multiple nodes on the same spot

Option 3:

In addition to that, JOSM validator will warn you regarding many unconnected nodes without physical characteristics in that area - and even fix them! That's probably the best way to go.