Rated 4 out of 5 stars

it is disabled when be used on in-box,why?And does it support Simple Chinese?

This review is for a previous version of the add-on (0.3.2). 

removedupes skips by default all special folders except for the Inbox. However, for it to go through the Inbox, the Inbox' name and the name removedupes _thinks_ the Inbox has have to match. So if the localization is wrong or missing, removedupes will skip the Inbox. You can fix this by unchecking 'skip special folders', or setting the removedupes.allowed_special_folders pref to the appropriate regexp (e.g. ^blah$ if your Inbox' name is blah).

As for Chinese support - it works for Hebrew, so it should work for Chinese too.