fix: do not append duplicates to user agent string (#570)

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Fixes #565 🦕

google/cloud/storage/_http.py

@@ -56,7 +56,9 @@ def __init__(self, client, client_info=None, api_endpoint=None):
         # TODO: When metrics all use gccl, this should be removed #9552
         if self._client_info.user_agent is None:  # pragma: no branch
             self._client_info.user_agent = ""
-        self._client_info.user_agent += " gcloud-python/{} ".format(__version__)
+        agent_version = "gcloud-python/{}".format(__version__)
+        if agent_version not in self._client_info.user_agent:
+            self._client_info.user_agent += " {} ".format(agent_version)
 
     API_VERSION = "v1"
     """The version of the API, used in building the API call's URL."""

tests/unit/test__http.py

@@ -228,3 +228,18 @@ def test_mtls(self):
         self.assertEqual(conn.ALLOW_AUTO_SWITCH_TO_MTLS_URL, False)
         self.assertEqual(conn.API_BASE_URL, "http://foo")
         self.assertEqual(conn.API_BASE_MTLS_URL, "https://storage.mtls.googleapis.com")
+
+    def test_duplicate_user_agent(self):
+        # Regression test for issue #565
+        from google.cloud._http import ClientInfo
+        from google.cloud.storage.client import Client
+        from google.cloud.storage.batch import Batch
+        from google.cloud.storage import __version__
+
+        client_info = ClientInfo(user_agent="test/123")
+        client = Client(project="test-project", client_info=client_info)
+        conn = self._make_one(client, client_info)
+        expected_user_agent = "test/123 gcloud-python/{} ".format(__version__)
+        self.assertEqual(conn._client_info.user_agent, expected_user_agent)
+        batch = Batch(client)
+        self.assertEqual(batch._client_info.user_agent, expected_user_agent)