send_order_notice: use builtin set() functionality
send_order_notice: use builtin set() functionality

That way, we don't have to roll our own duplication checks.

Ryati/satchmo,twidi/satchmo,ringemup/satchmo,dokterbob/satchmo,ringemup/satchmo,Ryati/satchmo,twidi/satchmo,dokterbob/satchmo