Hi there.
My first thought is to try playing co-op with another person who doesn't have the achievement and collect the items again.
I tried to hop on over to lionhead.com and check out the forums. Imagine my surprise when the support forums were no more. There is an ask lionhead comment page now located here
lionhead.com/ask-lionhead-the-answers . I would suggest that you also comment about your question over there. The contact us support page does lead you to these xbox forums. So I checked around some trusted net sources and found some links that may be
helpful to you .
Maybe the folks over at xbox 360 achievements could help you double check:
www.xbox360achievements.org/.../forumdisplay.php
www.xbox360achievements.org/.../51132-A-Revolutionary-Idea.html
www.xbox360achievements.org/.../51131-Lab-Notes.html
Hope this helps you out.