multi-column index has 1 unique column vs single column let assume that I have 2 columns 

comment_id | unique
shop_name  | duplicated


which case is batter ? 

1- making multi-column index using these 2 columns 

2- making single column index using comment_id

which one is batter based on your experience?
I would suggest both,


Making comment_id a PRIMARY KEY: which is both UNIQUE and NOT NULL
If shop_name is frequently a part of the query, making shop_name it's own index.