SNS: do not duplicate subscriptions

moto/sns/models.py

@@ -247,11 +247,21 @@ class SNSBackend(BaseBackend):
         setattr(topic, attribute_name, attribute_value)
 
     def subscribe(self, topic_arn, endpoint, protocol):
+        # AWS doesn't create duplicates
+        old_subscription = self._find_subscription(topic_arn, endpoint, protocol)
+        if old_subscription:
+            return old_subscription
         topic = self.get_topic(topic_arn)
         subscription = Subscription(topic, endpoint, protocol)
         self.subscriptions[subscription.arn] = subscription
         return subscription
 
+    def _find_subscription(self, topic_arn, endpoint, protocol):
+        for subscription in self.subscriptions.values():
+            if subscription.topic.arn == topic_arn and subscription.endpoint == endpoint and subscription.protocol == protocol:
+                return subscription
+        return None
+
     def unsubscribe(self, subscription_arn):
         self.subscriptions.pop(subscription_arn)
 

tests/test_sns/test_subscriptions_boto3.py

@@ -25,6 +25,23 @@ def test_subscribe_sms():
     )
     resp.should.contain('SubscriptionArn')
 
+@mock_sns
+def test_double_subscription():
+    client = boto3.client('sns', region_name='us-east-1')
+    client.create_topic(Name="some-topic")
+    resp = client.create_topic(Name="some-topic")
+    arn = resp['TopicArn']
+
+    do_subscribe_sqs = lambda sqs_arn: client.subscribe(
+        TopicArn=arn,
+        Protocol='sqs',
+        Endpoint=sqs_arn
+    )
+    resp1 = do_subscribe_sqs('arn:aws:sqs:elasticmq:000000000000:foo')
+    resp2 = do_subscribe_sqs('arn:aws:sqs:elasticmq:000000000000:foo')
+
+    resp1['SubscriptionArn'].should.equal(resp2['SubscriptionArn'])
+
 
 @mock_sns
 def test_subscribe_bad_sms():