Thunderbird Version: 18.104.22.168 (20090812)
Some mails appear twice in the inbox. Other are not visible at all but can be found when searching for their subject (which requires to know the subject in the first place ;)
Does anybody know a fix for this?
This seems to be a known problem that just happens from time to time. As Abhinav suggests the solution seems to be to recreate the index. I've found more infos here: