Double opt-in disabled yet double opt-in email sent

Hi,
I’m currenty having an issue with a double opt-in email that is sent to the contacts that subscribe to the newsletter on my homepage.
The email is automatically sent, but i haven’t enabled the double opt-in on my website.
Also the object of the email shows the name of the Mailchimp audience the contacts are being signed to. I don’t know where to configurate this email.
I have already talked to the Mailchimp support and they have are not responsible for this email, since an external API is sending the data to Mailchimp.
Does anybody know where to configurate this email and how to disable it?
Thanks in advance,
Cheers?

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