In Rhino, it is possible to have a layer structure in which different parent layers have identical sublayer names. This is common in situations where you have multiple design options, each as a separate layer but their sublayers have the same structure. In Unreal, when these are imported the sublayers with identical names get a number appended to them due to identical actor names in the world outlines. However, it is possible to manually edit the actors names to be identical again, so it doesn't seem to be a hard restriction.
This was tested and reproduced in 4.20 (4.20E4) and 4.21 (4.21E0). In 4.19.2 (0.182) it just imports a blank scene
Results: Imported sublayers with identical names get a number appended to them
Expected: No changes made to identical sublayers
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-65251 in the post.