Menu -> information -> assemblies -> list components will report if you have overlaid, identical components. There will be one or more lines at the bottom of the report that looks like:
The report will list the name of the component, but I cut it out of the screenshot.
there is an old Grip program that detects and deletes overlapping entities, but it only works
for points, lines and circles, and cannot delete occurrences.
A journal could be written if wireframe only is needed, but identifying duplicate solids seems
more tricky to me.
Apart from this, the VDA4955 Checker add-on module can identify duplicate geometries.
Here is a journal to help find duplicate single face sheet bodies. It isn't bullet proof, but it works reasonably well.
Note: it works on a piece part file, not an assembly as mentioned in the opening post.