Pretend you onboard customer 1 and they name their PCs stuff like "John's PC"

Then you onboard customer 2, who does the same thing and has someone named John also.

"Sorry, we can't allow a customer to control the hostname of their machine"?

"Sorry, no John's allowed to work here anymore"?

What's your approach to this?

Better idea - stop using hostnames as a identifier.

Merge duplicate serial numbers