Any role that would receive a result and the double neighborizers will be explicitly informed if their abilities fail. The echo will be told that they didn't receive any ability (which they will also be told if they target someone who did not use anything). No other roles are informed.
Gunsmith: will return positive on mafia, vanilla vig, power vig, paranoid vig, dayvig, or an echo who has copied any of those abilities (it will resolve after the echo).
Follower: will learn whatever was used,
including
the mafia kill. If that player made a ninja kill that night, they will be told the player did not use an ability.
Power Doctor/Power Vigilante: these roles do not need to specify
which
copy of a role was won (so you can target implosion and Tracker, rather than implosion and Tracker B).
Ninja/Strongman: if the same mafia member possesses both of these, they may be used in conjunction on one kill.
Echo: can copy anything except for the mafia kill, strongman and ninja. They will copy the action their target took even if that action fails.