Hi Tris,
I just had the same problem with Silverstripe 2.4.1.
Same as you I followed the steps at http://doc.silverstripe.org/widgets#adding_widgets_to_other_pages to enable Widgets on all pages which works fine, but I also ended up with a doubled-up widgets area editor on blog holder pages in the CMS.
Here is a workaround. Maybe it's not the ideal way but it seems to work. In /mysite/page.php in the Page class where you defined the WidgetAreaEditor for the interface (as described in the link above) simply make sure that Silverstripe does not do this for the BlogHolder page type because this causes the double WidgetAreaEditor. Here is the if statement I added (in green):
if($this->ClassName != 'BlogHolder') {
$fields->addFieldToTab('Root.Content.Widgets', new WidgetAreaEditor('Sidebar'));
}
In versions prior to 2.4 I could simply remove the WidgetAreaEditor from BlogHolder.php but that doesn't seem to work properly any more since the WidgetAreaEditor was moved to BlogTree.php .
I hope that helps.
Cheers!
Anatol