Usually you'll get a notice that somebody's added you before you've added them in the LINE chat app. But if you've blocked/deleted people but they have you on their list and send to you, they'll show up as unknown. If you accept each "unknown" heart individually, it'll send a heart back to them automatically. Then you can click "claim all" for your knowns and send down your list to them. (There is a limit of 50 heart notifications so if you have an active list, you may have hidden unknowns further down that you can't see yet.)