Amazon SNS multiple subscriptions of the same endp

问题:

I noticed that if I try to create a subscription between a topic and a SQS queue twice, Amazon actually creates only a single one.

aws sns subscribe --topic-arn arn:aws:sns:ap-southeast-2:909420746768:bar --protocol sqs --notification-endpoint arn:aws:sqs:ap-southeast-2:909420746768:foo --region=ap-southeast-2
{
    "SubscriptionArn": "arn:aws:sns:ap-southeast-2:909420746768:bar:39d8296e-18e3-4367-b10b-edd5af266f69"
}

-- if I call this command twice, the same SubscriptionArn is returned each time. If I call list-subscriptions, it will return a list with the single item too.

Cannot find the description of this behavior anywhere in Amazon's documentation, is it mentioned somewhere? Is it uniform across all protocols?

Thank you.