firestore cloud functions onCreate/onDelete sometimes immediately triggered twice

Question

I have observed this behavior occasionally with both onCreate and onDelete triggers.

Both the executions happened for the same document created in firestore. There

Answer 1

See the Cloud Firestore Triggers Limitations and Guarantees:

Delivery of function invocations is not currently guaranteed. As the Cloud Firestore and Cloud Functions integration improves, we plan to guarantee "at least once" delivery. However, this may not always be the case during beta. This may also result in multiple invocations for a single event, so for the highest quality functions ensure that the functions are written to be idempotent.

There is a Firecast video with tips for implementing idempotence.

Also two Google Blog posts: the first, the second.