Warn user of duplicate official name

[vinkabuki]

No description provided.

[kingalg]

Link to figma designs: https://www.figma.com/file/TV9pF84Ob8pLYRLu83gNol/Joining-when-owner-is-offline?type=design&node-id=7-2510&mode=design

Screenshot of designs from figma:

Details as per discussion in #1340:

• When there are two or more official users with the same name, we should show a very aggressive full screen warning such as "Warning: impersonation attack. The community owner has given two people the same name. This should never happen, and it means the owner is malicious or compromised."
• "Warning: impersonation attack" should appear "If the owner hands out duplicate names (which should never happen) it should be very visible. On every restart, for example." @holmesworcester as we are about to work on this issue could you confirm:

1. if this should happen on every restart?
2. are there any other situations in which it should appear?

[holmesworcester]

It should show as soon as we notice it.

On desktop it should show on every restart, but on mobile 'restart' means something different.

I think it makes sense to redesign this to be an alert banner that displays all the time rather than a full screen thing. Or it could be a full screen warning followed by an alert banner that displays all the time. Let's knock this back to 'needs design' and move it out of the sprint.

[holmesworcester]

duplicate of #119. closing.