Maybe they could introduce an achievement in the style of "Talk to Jonathan wearing the full 2B set" which will unlock the set from the Calamtiy Salvager but will prevent you from rolling on the coffer again?

Its not perfect, since it wont protect against trolls who could simply not bother to get the achievment and keep happly rolling, but since the game seems to have no real way to check wether or not you own a certain piece of gear somewhere its the best I could think of... would also be a nice addition so people could safely discard the set and still have an easy way of getting it back.