register each yubikey only once about webauthn HOT 2 OPEN

There isn't currently, and to do it properly would require some kind of unique id for the device. Obviously that's related to device identification which you also commented on.

Having said that, it doesn't really matter if the device is registered more than once.

There is a pull request outstanding about identifying devices which I haven't had a chance to look at yet, but there the device is being named by the user.

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I have been test it work ok .
maybe you can use this link :
#14
if this user use the fido device >1,
error info : InvalidStateError: An attempt was made to use an object that is not, or is no longer, usable

from webauthn.