No, each web site has its own public/private key pair, they're just derived from the same original seed (plus scoping information like the user and site IDs). Y

No, each web site has its own public/private key pair, they're just derived from the same original seed (plus scoping information like the user and site IDs). You can't determine if two different users are using the same identifier, although you can determine if they're using hardware tokens from the same manufacturer and batch. U2F's mechanism to check the token is from a "safe" manufacturer provides some information about the token, but not its exact serial number or other unique ID.