Duplicate messages in Thunderbird

I started using Thunderbird this week and noticed that on USENET, many threads appear to have double posts with the same sende/date/time. Some threads appear normal but many are not. Here is a link to a screenshot of what I'm looking at. (100kB)
Notice the thread titled (Linux Device Drivers)appears normal while the one above it titled (java runtime editor with firefox) has duplicate posts.

I did not have this problem with Debian's stable Mozilla 1.7.8 release. Is anyone else having this problem? or what am I doing wrong?