According to this article https://docs.mongodb.com/manual/reference/method/db.collection.save/

save does not support an array of documents but I could insert multiple documents as follows

db.test.save(
	[
		{
			"_id": 1,
			"fruit": "apple"  	
		},
		{
			"_id": 2,
			"fruit": "banana"  	
		}
	]
)

Now if I run the below command to add '123' at the end of the fruit names, then the save throws duplicate key error. In fact this should have been an upsert since the '_id's are the same.

db.test.save(
	[
		{
			"_id": 1,
			"fruit": "apple 123"  	
		},
		{
			"_id": 2,
			"fruit": "banana 123"  	
		}
	]
)

If I save just a single document then the upsert works as expected

db.test.save(
	{
		"_id": 1,
		"fruit": "apple 123"  	
	}
)

Why is it allowed to upsert multiple documents?