diff --git a/moto/iam/models.py b/moto/iam/models.py
index 4a115999c..81c7d09ca 100644
--- a/moto/iam/models.py
+++ b/moto/iam/models.py
@@ -733,6 +733,10 @@ class IAMBackend(BaseBackend):
         policy = ManagedPolicy(
             policy_name, description=description, document=policy_document, path=path
         )
+        if policy.arn in self.managed_policies:
+            raise EntityAlreadyExists(
+                f"A policy called {policy_name} already exists. Duplicate names are not allowed."
+            )
         self.managed_policies[policy.arn] = policy
         return policy
 
diff --git a/tests/test_core/test_auth.py b/tests/test_core/test_auth.py
index 7dc632188..c5e043ae5 100644
--- a/tests/test_core/test_auth.py
+++ b/tests/test_core/test_auth.py
@@ -402,10 +402,10 @@ def test_s3_access_denied_with_denying_attached_group_policy():
         "Statement": [{"Effect": "Deny", "Action": "s3:List*", "Resource": "*"}],
     }
     access_key = create_user_with_access_key_and_attached_policy(
-        user_name, attached_policy_document
+        user_name, attached_policy_document, policy_name="policy1"
     )
     create_group_with_attached_policy_and_add_user(
-        user_name, group_attached_policy_document
+        user_name, group_attached_policy_document, policy_name="policy2"
     )
     client = boto3.client(
         "s3",
@@ -476,10 +476,16 @@ def test_access_denied_with_many_irrelevant_policies():
         "Statement": [{"Effect": "Deny", "Action": "lambda:*", "Resource": "*"}],
     }
     access_key = create_user_with_access_key_and_multiple_policies(
-        user_name, inline_policy_document, attached_policy_document
+        user_name,
+        inline_policy_document,
+        attached_policy_document,
+        attached_policy_name="policy1",
     )
     create_group_with_multiple_policies_and_add_user(
-        user_name, group_inline_policy_document, group_attached_policy_document
+        user_name,
+        group_inline_policy_document,
+        group_attached_policy_document,
+        attached_policy_name="policy2",
     )
     client = boto3.client(
         "ec2",
diff --git a/tests/test_iam/test_iam.py b/tests/test_iam/test_iam.py
index c4fcda317..9a34bbf6a 100644
--- a/tests/test_iam/test_iam.py
+++ b/tests/test_iam/test_iam.py
@@ -400,6 +400,21 @@ def test_create_policy():
     )
 
 
+@mock_iam
+def test_create_policy_already_exists():
+    conn = boto3.client("iam", region_name="us-east-1")
+    response = conn.create_policy(
+        PolicyName="TestCreatePolicy", PolicyDocument=MOCK_POLICY
+    )
+    with assert_raises(conn.exceptions.EntityAlreadyExistsException) as ex:
+        response = conn.create_policy(
+            PolicyName="TestCreatePolicy", PolicyDocument=MOCK_POLICY
+        )
+    ex.exception.response["Error"]["Code"].should.equal("EntityAlreadyExists")
+    ex.exception.response["ResponseMetadata"]["HTTPStatusCode"].should.equal(409)
+    ex.exception.response["Error"]["Message"].should.contain("TestCreatePolicy")
+
+
 @mock_iam
 def test_delete_policy():
     conn = boto3.client("iam", region_name="us-east-1")