Raise DuplicateOrganizationalUnitException

Calling UpdateOrganizationalUnit with name that already exists should
raise proper error.

moto/organizations/exceptions.py

@@ -10,3 +10,13 @@ class InvalidInputException(JsonRESTError):
             "InvalidInputException",
             "You provided a value that does not match the required pattern.",
         )
+
+
+class DuplicateOrganizationalUnitException(JsonRESTError):
+    code = 400
+
+    def __init__(self):
+        super(DuplicateOrganizationalUnitException, self).__init__(
+            "DuplicateOrganizationalUnitException",
+            "An OU with the same name already exists.",
+        )

moto/organizations/models.py

@@ -8,7 +8,10 @@ from moto.core import BaseBackend, BaseModel
 from moto.core.exceptions import RESTError
 from moto.core.utils import unix_time
 from moto.organizations import utils
-from moto.organizations.exceptions import InvalidInputException
+from moto.organizations.exceptions import (
+    InvalidInputException,
+    DuplicateOrganizationalUnitException,
+)
 
 
 class FakeOrganization(BaseModel):
@@ -223,6 +226,9 @@ class OrganizationsBackend(BaseBackend):
         return new_ou.describe()
 
     def update_organizational_unit(self, **kwargs):
+        for ou in self.ou:
+            if ou.name == kwargs["Name"]:
+                raise DuplicateOrganizationalUnitException
         ou = self.get_organizational_unit_by_id(kwargs["OrganizationalUnitId"])
         ou.name = kwargs["Name"]
         return ou.describe()