Turn off Mailchimp Double Opt-in

Also, how can I show a message if a user is already signed up?

Yes it's actually turned off by default. You'd need to disable it in MailChimp too of course. Double Opt-in is recommended for nice clean mailing lists though.

Any error reporting will come back from MailChimp's API including duplicate sign ups.

Failing that you can just embed MailChimp's own forms.

It's all in the docs. https://docs.grabaperch.com/addons/mailchimp/examples/subscription-form/

Ok perfect. Got that working.

Now... How would I go about allowing members to re-subscribe after they have unsubscribed? Would I have to do this manually?

This is the debug which clearly shows I am on the mailing list already - but I have chosen to unsubscribe so my status shows this.

Array
(
    [headers] => Array
        (
            [url] => https://us16.api.mailchimp.com/3.0/lists/253305ee17/members
            [content_type] => application/problem+json; charset=utf-8
            [http_code] => 400
            [header_size] => 421
            [request_size] => 449
            [filetime] => -1
            [ssl_verify_result] => 0
            [redirect_count] => 0
            [total_time] => 0.153995
            [namelookup_time] => 0.004252
            [connect_time] => 0.005888
            [pretransfer_time] => 0.022598
            [size_upload] => 114
            [size_download] => 195
            [speed_download] => 1266
            [speed_upload] => 740
            [download_content_length] => 195
            [upload_content_length] => 114
            [starttransfer_time] => 0.153854
            [redirect_time] => 0
            [redirect_url] => 
            [primary_ip] => 23.64.21.120
            [certinfo] => Array
                (
                )

            [primary_port] => 443
            [local_ip] => 175.2.4.102
            [local_port] => 64494
            [request_header] => POST /3.0/lists/253305ee17/members HTTP/1.0
Host: us16.api.mailchimp.com
User-Agent: DrewM/MailChimp-API/3.0 (github.com/drewm/mailchimp-api)
Accept-Encoding: deflate, gzip
Accept: application/vnd.api+json
Content-Type: application/vnd.api+json
Authorization: apikey 06abffa6459ec6853c95e5bc49d49324-us16
Content-Length: 114

        )

    [body] => {"type":"https://developer.mailchimp.com/documentation/mailchimp/guides/error-glossary/","title":"Member Exists","status":400,"detail":"a.person@icloud.com is already a list member. Use PUT to insert or update list members.","instance":""}
)

Ok.. but.. what if the user chose to unsubscribe but now wants back on the list? Would I then manually have to resend the subscribe email?

How can I echo back an error message on the users end subscribe form so they know they need to contact me?

This is a MailChimp issue, of which there are several solutions depending on how you set up your mailing list. You just need to read the docs!

You can resubscribe users manually or they can sign up again through the form so my previous comment was a bit misleading.

https://kb.mailchimp.com/lists/manage-contacts/resubscribe-a-contact