Check Email Canonical

Check whether a Gmail address is a dot-trick duplicate of another account.

Only meaningful for @gmail.com / @googlemail.com; every other domain answers duplicate: false, as does the submitter's own address — a returning user is not their own duplicate. The unauthenticated route is IP-rate-limited.

Args: request: Incoming request, required by the rate limiter. body: Full address submitted at signup.

Returns: Whether a different address canonicalises to the submitted one.