deleting reference and importing objects causes 1000s of design layers etc

[ASL]

In trying to 'save' a file, which kept causing VW to crash, I decided to reference said file into a blank document and then to delete the reference, but asking for the objects to be 'imported into current document'.

This caused apx. 3000 design layers to develop (whereas there used to only be one) and each class to duplicate anywhere from 10 to 20 times.

What could be causing this?

I tried this again with a much simpler file and it only added one additional design layer and half a dozen classes, but why would this be happening at all?