Makes Creating SNS topics idempotent (#1324)

Make creating SNS topics idempotent

Closes #1323

moto/sns/models.py

@@ -193,9 +193,12 @@ class SNSBackend(BaseBackend):
         self.sms_attributes.update(attrs)
 
     def create_topic(self, name):
-        topic = Topic(name, self)
-        self.topics[topic.arn] = topic
-        return topic
+        candidate_topic = Topic(name, self)
+        if candidate_topic.arn in self.topics:
+            return self.topics[candidate_topic.arn]
+        else:
+            self.topics[candidate_topic.arn] = candidate_topic
+            return candidate_topic
 
     def _get_values_nexttoken(self, values_map, next_token=None):
         if next_token is None:

tests/test_sns/test_topics_boto3.py

@@ -31,6 +31,29 @@ def test_create_and_delete_topic():
     topics = topics_json["Topics"]
     topics.should.have.length_of(0)
 
+@mock_sns
+def test_create_topic_should_be_indempodent():
+    conn = boto3.client("sns", region_name="us-east-1")
+    topic_arn = conn.create_topic(Name="some-topic")['TopicArn']
+    conn.set_topic_attributes(
+        TopicArn=topic_arn,
+        AttributeName="DisplayName",
+        AttributeValue="should_be_set"
+    )
+    topic_display_name = conn.get_topic_attributes(
+        TopicArn=topic_arn
+    )['Attributes']['DisplayName']
+    topic_display_name.should.be.equal("should_be_set")
+
+    #recreate topic to prove indempodentcy
+    topic_arn = conn.create_topic(Name="some-topic")['TopicArn']
+    topic_display_name = conn.get_topic_attributes(
+        TopicArn=topic_arn
+    )['Attributes']['DisplayName']
+    
+    topic_display_name.should.be.equal("should_be_set")
+
+
 
 @mock_sns
 def test_get_missing_topic():