The addon now syncs Google Groups with Thunderbird, SeaMonkey, Postbox and Spicebird addressbooks. The feature is available in release 0.8.15 and later.
If the addon really created duplicates it’d be a serious bug. Neither of these folk filed a bug report so it’s impossible to know for sure.
But while bugs are always possible, it’s far more likely that these reports reflect a misalignment between expectations and the reality of slow sync.