Existing subscriber can sign up even if "Update Existing Subscriber?" is turned off

[MaxwellGarceau]

Describe the bug

An existing subscriber can sign up with the same email and not receive the expected "This email address is already subscribed to the list."

Introduced with == loose equality to === strict equality change.

Steps to Reproduce

1. Turn off "Update Existing Subscriber?"
2. Sign up with an email address
3. Use the same email address to sign up again
4. The submission will succeed where it should have failed

Screenshots, screen recording, code snippet

subscriber-already-exists-check-ignored.mov

Environment information

No response

WordPress information

No response

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