Post from their Community Manager here.
Servers are offline whilst they attempt to remove all duped items from player inventories and stashes.
I guess that could include items that were freely traded or exchanged for other goods / caps but were originally created through duplication.
Iām unsure if all duping methods have been patched. Regardless, this generally seems positive.
Edit: Servers are still offline as of 12:00 EST (17:00 GMT).