An oversized comment only
- how this would work in a modern version of Illustrator
- what's the detection method and tolerance
Then there's commercial solutions. Astute VectorFirstAid is advertised to do the job (never got one).
These were found by making a web search for an Illustrator script which removes duplicates.
All automatic methods may fail if you have splinters which do not have equal anchor points, they only happen to be visually in the same place. Manual removal is easier if you make the items partially transparent:
In the right the opacities are reduced to 50%. The redundant piece is darker.
Another way to make the redundant piece visible is to select a path and move it:
The ellipse is moved one arrow key tick downwards (the step size is in preferences). It returns back exactly by moving it one tick upwards.