google cloud platform - Prevent duplicate entries in Firestore rules not working

Question

I'm trying to prevent duplicate entries using Google Firestore rules, however it's not working. The rule I'm trying is:

service cloud.firestore {  
  // Prevent duplicate messages
  match /databases/{database}/documents {
    match /messages/{message} {
        allow read;
      allow write: if request.resource.data.m != resource.data.m;
    }
  }
}

From what I read, this should work.

What am I doing wrong?

Answer 1

Waitting for answers