Mailchimp duplicate confirmation emails

I've set-up the Mailchimp app with a Runway site today but my subscribe form seems to be causing MC to send out two "confirm-your-email-address" emails to every subscriber.

My short diagnostics report:

Perch Runway: 2.8.23, PHP: 5.4.36, MySQL: mysqlnd 5.0.10 - 20111026 - $Id: c85105d7c6f7d70d609bb4c000257868a40840ab $, with PDO
Server OS: Linux, cgi-fcgi
Installed apps: content (2.8.23), assets (2.8.23), categories (2.8.23), dh_ga (1), perch_blog (5.0), perch_forms (1.8.3), perch_gallery (2.8.6), perch_mailchimp (2.0.1), collection_3 (2.8.23), collection_2 (2.8.23), collection_1 (2.8.23)
App runtimes: <?php $apps_list = array( 'content', 'categories', 'perch_blog', 'perch_gallery', 'perch_forms', 'perch_mailchimp', ); ?>
PERCH_LOGINPATH: /perch
PERCH_PATH: /home/cvwdesign/public_html/perch
PERCH_CORE: /home/cvwdesign/public_html/perch/core
PERCH_RESFILEPATH: /home/cvwdesign/public_html/perch/resources
Image manipulation: GD
PHP limits: Max upload 8M, Max POST 8M, Memory: 128M, Total max file upload: 8M
F1: dc1fef2ad0fcd9f943c02ebb43d85dbc
Resource folder writeable: Yes
SCRIPT_NAME: /perch/core/settings/diagnostics/index.php
REQUEST_URI: /perch/core/settings/diagnostics/
DOCUMENT_ROOT: /home/cvwdesign/public_html
HTTP_HOST: www.cvwdesign.co.uk

My subscribe.html template

<perch:form id="subscribe" app="perch_mailchimp" double-optin="true" send-welcome="true" class="comment-form" >

    <perch:success>
        <p class="success">Thank you! You should get an email shortly with a link to confirm your email address. Please check your spam folder if this doesn't arrive within a few minutes.</p>
    </perch:success>
<fieldset>
    <div>
        <perch:label for="email">Email</perch:label>
        <perch:input id="email" required="true" type="email" mailer="email" />
        <perch:error type="required" for="email">Required</perch:error>
    </div>
    <div>
        <perch:label for="firstname">First name</perch:label>
        <perch:input id="firstname" required="true" type="text" mailer="FNAME" />
        <perch:error type="required" for="firstname">Required</perch:error>
    </div>
    <div>
        <perch:label for="lastname">Last name</perch:label>
        <perch:input id="lastname" required="true" type="text" mailer="LNAME" />
        <perch:error type="required" for="lastname">Required</perch:error>
    </div>
    <div>
        <perch:input type="submit" value="Sign Up" id="btnsubmit" />
        <perch:input type="hidden" value="1" id="confirm" mailer="confirm_subscribe" />
    </div>  
    </fieldset>

</perch:form>

So, every subscriber now gets three emails, two for "confirm-your-email-address" and another welcome email.

Is there anything I should change here to get rid of the duplicate emails?

I may test the MC sign-up form code instead of the subscribe.html template as next step.