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:
http://clarkware.com/cgi/blosxom/2007/02/24

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.
html:
Test_thumb

but the other is:
Test_thumb_thumb

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