Block add bar shown twice

This is a small issue that I've had since the beta (I did email a screenshot at the time). When I save a block the 'block-add-bar' links show twice below each block. This happens locally and on live server, on different hosts and in Runway and Standard installations.

A very simple template produces the issue:

<perch:blocks >

<perch:block type="image" label="Image">
  <img src="<perch:content type="image" id="image" label="Image" width="800" />" alt="<perch:content type="text" id="alt" label="Description" required="true" title="true" />" />
</perch:block>

<perch:block type="textarea" label="Text Block">
    <perch:content id="text" type="textarea" label="Text" textile="true" editor="markitup" size="s" html="true" />
</perch:block>

</perch:blocks>

Here's my diagnostic:

Perch: 2.8.4, PHP: 5.4.25, MySQL: mysqlnd 5.0.10 - 20111026 - $Id: c85105d7c6f7d70d609bb4c000257868a40840ab $, with PDO
Server OS: Linux, cgi-fcgi
Installed apps: content (2.8.4), assets (2.8.4), categories (2.8.4), perch_events (1.9.1), perch_forms (1.8.2), perch_gallery (2.8.6), perch_blog (4.6), perch_twitter (3.5.1)
App runtimes: <?php $apps_list = array( 'content', 'categories', 'perch_gallery', 'perch_twitter', 'perch_events', 'perch_blog' );
PERCH_LOGINPATH: /perch
PERCH_PATH: /home/boblockw/public_html/ffrs/perch
PERCH_CORE: /home/boblockw/public_html/ffrs/perch/core
PERCH_RESFILEPATH: /home/boblockw/public_html/ffrs/perch/resources
Image manipulation: GD
PHP limits: Max upload 20M, Max POST 20M, Memory: 200M, Total max file upload: 20M
Resource folder writeable: Yes
DOCUMENT_ROOT: /home/boblockw/public_html/ffrs
HTTP_HOST: ffrs.boblockwood.co.uk
REQUEST_URI: /perch/core/settings/diagnostics/
SCRIPT_NAME: /perch/core/settings/diagnostics/index.php