Why are GlobalIds not unique across files?

For example, on a sample of 6 architectural Revit models on the same project (and therefore 6 IFC files), I found 2,578 duplicate GlobalIDs across elements from a total of 1,898,180 rooted elements. This is a duplication rate of 0.136% which is pretty severe.

This is creating lots of problems with linking to other systems. I have to resort to all sorts of tricks to regenerate and map changed GlobalIds.

What can be the cause of this? From the few I checked manually, the duplication happens across files, not within the same file. Perhaps copied elements?

The report names no source files, tests, or entry points. Start by reproducing the duplicate GlobalIds across the six referenced IFC files and trace how the Revit exporter assigns them, including whether copied elements are involved. Done means identifying the cause and documenting or correcting the behavior so GlobalIds are unique across exported files.

Written by the indexing model from the issue text.