4.1.2. "sub" (Subject) Claim
The "sub" (subject) claim identifies the principal that is the
subject of the JWT. The claims in a JWT are normally statements
about the subject. The subject value MUST either be scoped to be
locally unique in the context of the issuer or be globally unique.
The processing of this claim is generally application specific. The
"sub" value is a case-sensitive string containing a StringOrURI
value. Use of this claim is OPTIONAL.
https://www.rfc-editor.org/rfc/rfc7519#section-4.1.2要么同一个 issuer 唯一,要么全局唯一,这里 issuer 都是 google ,所以是唯一的