This extension has been effectively abandoned by its author for almost 2 years, without significant updates. It is also not free software, so the code cannot be used elsewhere.
For this reason I have fully rewritten a dupe removal extension of my own, which is actually free software:
Remove Duplicate Messages (alternate)
It has seen more active development, has a richer feature set (which proerly includes the features of this extension) and for which issues can be opened on Mozdev's bugzilla and addressed within a reasonable amount of time.