Duplicate content bug is caused mostly by two things:
In IE6 (not sure about IE7?) you can cause them by having a comment coming directly after a float, or a comment in between floats. It has to do with IE confusing comments with real tags.
For now, remove ALL comments or you'll never know if they were the cause.
However I've managed to trigger duplicate content bug in IE (6 and 7) by having a setup where I had block elements next to inline or inline-block elements.
Hm, a shame, I don't seem to have the screenshots on my server anymore. I may make another. But here's what I encountered and how I fixed it.