LoginNotify issues multiple notifications if there have been multiple login attempts. Solving this could be somewhat tricky because, if I understand correctly, once you issue an Echo notification, it's done and can't be modified. So we'd likely have to settle for a single notification for each login attempt. Or the current thing.
Bundle the notifications. Make the bundle say something like "There have been multiple failed attempts to log into your account...". Leave the unknown device threshold at 2. So the bundled notifications should say 2, 3, 4, etc. attempts.