Q:

In Firestore can there be duplicates of FieldValue.serverTimestamp()?

Imagine you have thoiusands, even millions of users connecting simmultaneously on your Firebase app. Your app (on every single user) write this to Firestore:
const ref = firestore.collection('users').doc(myUID)
ref.set({
    updatedAt: firebase.firestore.FieldValue.serverTimestamp()
})

Is it possible (even if very improbable) that more than one user has the same timestamp in updatedAt?

A:

There is no guarantee of uniqueness for server timestamps.  That said, I imagine it's highly unlikely to get a duplicate, since they are measured to nanosecond precision.

