Hey Chengbo, I think the Okta username which uses the email address by default must be unique, but the primary email address does not need to be unique by default. I’ve got multiple accounts with different usernames and the same primary email address (as you found) - take a look at my screenshot.
Usernames must be unique in Okta and most identity platforms, but not emails.
I don’t know why its marked as UNIQUE=TRUE in the referenced table. That page links to the SCIM standard which has a multi-valued field ‘emails’ which are only a SHOULD (not MUST).
I’d suggest its possibly a documentation error - unless user creation via the API has different constraints to the Okta Admin UI. I don’t have postman handy to check.