MailChimp API: "is already a list member. Use PUT to insert or update list members"
To upsert a list member, PUT to lists/{list_id}/members/{subscriber_hash} where the hash is md5(strtolower($email)) (the drewm/mailchimp-api wrapper provides $MailChimp->subscriberHash($email)). Pattern: GET that URL first; if the wrapper reports success, patch() it to update, otherwise post() to the collection URL to…
Problem outcome: unknown