[imp] some attachments are shown two times in message window

[Fabio Pedretti]

Hi,
I am using I am using Horde 3.1.5 with Imp 4.1.5. On some message with  
attachments the attachments are shown two times as in this screenshot:
http://bugs.horde.org/view.php?actionID=view_file&type=png&file=imp-attachment.png&ticket=5780

I reported it as a bug here:
http://bugs.horde.org/ticket/?id=5780
but the answer was that this is the expected behavior.

So, I would ask:
1) why this is enabled?
2) what's the element that make some attachments showed two times  
(only some attachments have this behavior)?
3) how to eventually disable this?

Thanks,
Fabio