This is a just a minor interface bug, but nonetheless a bug.

Problem: TestUsers gets created when enabling 3D sound.

How to reproduce:
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1. Open 3D sound setup for a channel.
2. Add one or more test users by clicking on 'Add Test User'.
3. (Re)Check 'Enable 3D sound'.

Everytime you repeat step 3 you get (N*2)-N new test users, where N = number of current test users. In other words; you get a new copy of every test user that you currently have.