Rated 3 out of 5 stars
EDIT: breaks for some cases with large folders / large messages. (Plugin will hang, and TBird must be killed.) Also gets confused by differences in some client-side headers (X-Antispam, in my case). For now, I'm using a combination of this version and the original Remove Duplicates, in order to work around these problems.
Will update this review when I have more info.
Abandoned? Not really... I'm releasing a new version soon actually (no major changes, mind you). You should try actually talking to people or having a look at the Bugzilla before declaring thing to be abandoned. Please consider retracting that.
Anyway, about the "taking a long time" experience - I haven't gotten such complaints. Can you open an issue at www.mozdev.org/bugs/ so we can get to the bottom of this?