Try to address the concurrency problem

app/models/member.rb

@@ -104,8 +104,12 @@ class Member < ApplicationRecord
   end
 
   def self.regenerate_all_notifications
+    ActiveRecord::Base.transaction do
+      ActiveRecord::Base.connection.execute('LOCK members IN ACCESS EXCLUSIVE MODE')
+
       Member.all.each do |member|
         member.regenerate_notifications
       end
     end
   end
+end

app/models/notification.rb

@@ -4,10 +4,14 @@ class Notification < ApplicationRecord
   scope :scheduled_for_today, ->() { where(status: 'scheduled', to_be_sent_on: Date.today) }
 
   def self.send_scheduled_for_today
+    ActiveRecord::Base.transaction do
+      ActiveRecord::Base.connection.execute('LOCK members IN ACCESS EXCLUSIVE MODE')
+
       scheduled_for_today.each do |n|
         n.deliver!
       end
     end
+  end
 
   def deliver!
     # actually send the email.