Change How MailChimp Add-on Handles Double Opt-in for Existing Subscribers

I truly believe “If a subscriber already exists, set them as subscribed” should be the default behaviour for the MailChimp Add-on:

Doing this will avoid the double opt-in for existing members.

Luckily I eventually happened to notice that this isn’t the default behaviour, but it isn’t obvious and makes me nervous about using the double opt-in.