There is very, very low chance two identical avatars can showed up in the world. It just happened to me when i join the world again and i saw two times same name.
One of friends also saw same bug. But most of them have not this bug.
My friend cannot see the double. Only some others can see them. But again very low chance.
But i want tell here i have never seen doubles in Live version, never in my life. Because the double will always kicked out if other same person is joining the same channel. So the security check works in Live.
But in Unity6 Beta, this security check has some flaw, allowing possible doubles in the world showing up.
The double is not moving but it's fully functional avatar and is clickable for information.
This means there is some mistake or missing checks that cause possible double avatars.
I suggest check the code and make sure the chance for doubles are not possible.
But again, this bug is very rare.

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