If put double signal means the car is already spoiled and "Stopped" ~~

but i've seen a number of car users on it when they drive on highway during really terribly heavy rain season ~~ tat's for safety to increase visibility~

for your case, actually there's no rule that says cannot on hazard light to indicate that the car is having some problem and thus moving very slowly ~
though it might cause some confusion for other road users, it will warn them beforehand so that they no need to horn their car ~~

IMO, better on the hazard like to give notification to other road users so that they know you dun slow down on purpose ~~