Thunderbird and gmail aren't exactly the best of friends. Gmail's tags mean that Thunderbird often downloads multiple copies of a single mail. Anything tagged in gmail will appear in a folder related to that tag, the "all mail" folder, and possibly the "inbox" and "sent mail" folders too. Thus a mail with multiple tags could potentially be stored more than four times in a local Thunderbird cache. This can make searching difficult, and is obviously wasteful of disk space.
The best solution I have come up with is as follows. Operate a zero inbox policy (i.e. use the inbox for processing live mail only and archive everything else) which eliminates an extra copy in the inbox. Secondly, configure Thunderbird not to sync the "Sent Mail" folder - this is a bit of a pain, since I actually find it quite useful to be able to look through just the mails I've sent, but a search can duplicate this functionality. In this way, most of the duplicates are removed, and only mail with tags is stored locally more than once.
Ideally, however, I'd only like one copy of each mail to be stored locally. I am surprised Thunderbird doesn't store mail by some sort of hashing algorithm to prevent precisely this problem - but it wouldn't be compatible with the way the folders are mirrored in a local directory structure, I suppose.
Can anyone think of a better way to get Thunderbird to cache a Google mail account locally efficiently.