One email daily?
I'm suggesting a notification option: one daily email (7 am user's local time) listing all the notifications from the previous day.
I turned off most of my daily email notifications because the barrage of all those followed discussions was suffocating my inbox. "Killing me softly, with your notifications..."
But five notifications in the bell is just too few.
So I propose a single daily email, the time relevant to each user -- heck, why not let each user choose the time of day relative say to GMT? One email, to help users keep track of notifications instead of being buried by them or missing them entirely.
Excellent idea! I think that's called a "digest." Duolingo should include a digest option with a choice of "no, send me all e-mails separately" "daily" and "weekly".
My email inbox has solved this on its own, as it chucks all the Duolingo emails into the spam folder. In the morning, I check my spam folder before my inbox!
But your idea is better than that of my email inbox... And GregHullender's is actually even better!