Document goes into specific collection.
Same JSON document can be stored in different collection with same key. If need to store in same collection you must use different key (key is unique in the collection).
bucket1.scope1.coll1 Key "k01" doc {"a":1}
bucket1.scope1.coll1 Key "k01" doc {"a":1} -- not allowed violates key (you can change key it allowed)
bucket1.scope1.coll2 Key "k01" doc {"a":1} -- Allowed
bucket1.scope2.coll1 Key "k01" doc {"a":1} -- Allowed
bucket2.scope1.coll1 Key "k01" doc {"a":1} -- Allowed