No feedback at all: if I see a known Alt account I won't tagged that user until the users does something fishy. I remember reading somewhere theymos said a user is free to create an alt account to drop a reply he/she doesn't want to be associated with his main account e.g political opinions. so giving that Alt account a neutral tag letting everyone know it's an alt account would had unmask that user and maybe put his life in danger.

I see no crime in having alt account until it's used for something evil/fishy then I will red tag.

One of the legitimate reason users create Alt account is because they can't afford 50+ BTC to have the privilege to change username.  I would love to change my username to something more unique and shorter but the best option now is to create an alt account but I'm scare not to get red tagged for owning one that's why I'm still using this account.