attachment_fu problem

I am getting a bizarre problem using attachment_fu and I was wondering
if anyone knows how to fix it/ why its happening.

Ive been following:

and I am up and running as far as upload etc goes, but when it comes
to display using:
<% for mugshot in @mugshots -%>
  <%= link_to image_tag(mugshot.public_filename(:thumb)),
mugshot.public_filename %>
<% end -%>

I get two images, one is a thumbnail which links to the original file.
That is what I want.
<a href="/image_uploads/0000/0001/test.JPG"><img alt="Test_thumb"
src="/image_uploads/0000/0001/test_thumb.JPG?1185027500" /></a>

but the other is:
<a href="/image_uploads/0000/0001/test_thumb.JPG"><img
alt="Test_thumb_thumb" src="/image_uploads/0000/0001/
test_thumb_thumb.JPG" /></a>

does anyone know where on earth the '_thumb_thumb' came from?
and does anyone know how to solve this problem?