We contacted MailerLite support and received the following answer:
I'm afraid we don't provide support when it comes to API - sorry about that! We do have an API overview document which may be useful, feel free to check it out here:
developers.mailerlite.com/docs
Unfortunately their API doc doesn't contain any description about double opt-in for existing users. We checked different parameters and each time it subscribes silently. It seems this implemented in that way on MailerLite side.
Regards,
ARI Soft