The uniqueness constraint for _id (ObjectID) is
per collection and _id (ObjectID) has to be unique in a collection, so YES! - One
and the same ObjectID can occur once per collection.
This is happening you would have to manually
insert duplicate ObjectID.
You can quickly verify this _id (ObjectID) by
inserting two documents with the same _id (ObjectID) in two different
collections.
For example, you can do it with
var id="_id";
docs = {...}; docs[id] = value;