Steps to reproduce:

1. Create new group in audio mixer
2. Right click and Duplicate group
3. Undo action, it doesn't get deleted, but made invisible
4. Undo again to destroy New group
5. New group - copy is now unreachable, undeletable.

Expected outcome: Undo deletes group duplicate.
Actual outcome: Undo makes duplicate group invisible, further undoing might make duplicate group unreachable unless you drag them from the asset window and remove them there.

Reproduced on 5.5.3p1, 5.6.0p4, 2017.1.0b4