Joplin is suited for huge number of notes because it is offline-first, data is store first in the local computer and it does synchronization gradually. Lots of mass import/export operations supported for multiple format.
Can you check this: How large can I grow my Joplin data and for how long?.
Regarding the duplicate email issue: currently each email received create a new note with a newly generated id. Do you mean spam emails, emails with similar content when you mention duplicate email?
This is a Proof of Concept, I'm still asking people whether they need this kind of self-hosting service, so I haven't invested deeply in it yet.