issue two purges for fastly surrogate keys

https://developer.fastly.com/learning/concepts/purging/#race-conditions states there are some race conditions we may be running into with shielding enabled.

ref: #12214

tests/unit/cache/origin/test_fastly.py

@@ -181,6 +181,14 @@ def test_purge_key_ok(self, monkeypatch):
         cacher.purge_key("one")
 
         assert requests_post.calls == [
+            pretend.call(
+                "https://api.fastly.com/service/the-service-id/purge/one",
+                headers={
+                    "Accept": "application/json",
+                    "Fastly-Key": "an api key",
+                    "Fastly-Soft-Purge": "1",
+                },
+            ),
             pretend.call(
                 "https://api.fastly.com/service/the-service-id/purge/one",
                 headers={
@@ -190,7 +198,7 @@ def test_purge_key_ok(self, monkeypatch):
                 },
             )
         ]
-        assert response.raise_for_status.calls == [pretend.call()]
+        assert response.raise_for_status.calls == [pretend.call(), pretend.call()]
 
     @pytest.mark.parametrize("result", [{"status": "fail"}, {}])
     def test_purge_key_unsuccessful(self, monkeypatch, result):

warehouse/cache/origin/fastly.py

@@ -10,6 +10,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import time
 import urllib.parse
 
 import requests
@@ -106,3 +107,11 @@ def purge_key(self, key):
 
         if resp.json().get("status") != "ok":
             raise UnsuccessfulPurgeError(f"Could not purge {key!r}")
+
+        time.sleep(2)
+
+        resp = requests.post(url, headers=headers)
+        resp.raise_for_status()
+
+        if resp.json().get("status") != "ok":
+            raise UnsuccessfulPurgeError(f"Could not double purge {key!r}")