Only lookup broadcasts which can be cancelled

It is possible that, among the references Environment Agency give us for
which broadcast to cancel, there could be references for older, already
expired broadcasts.

This would be the case if someone cancelled a broadcast in Notify, then
issued and try to re-cancel another broadcast to the same area. The
Flood Warning Service has no way of knowing that the first broadcast has
been cancelled in Notify already, so it would add the reference to the
list of things to be cancelled.

We can avoid this from happening by filtering-out already-cancelled and
expired broadcasts before looking up which one should be cancelled.

tests/app/v2/broadcast/test_post_broadcast.py

@@ -253,6 +253,56 @@ def test_cancel_request_does_not_cancel_broadcast_if_service_id_does_not_match(
     assert response_for_cancel.status_code == 404
 
 
+@pytest.mark.parametrize("is_approved, expected_cancel_tasks", (
+    (True, 1),
+    (False, 0),
+))
+def test_same_broadcast_cant_be_cancelled_twice(
+    mocker,
+    client,
+    sample_broadcast_service,
+    is_approved,
+    expected_cancel_tasks,
+):
+    mock_send_broadcast_event_task = mocker.patch(
+        'app.celery.broadcast_message_tasks.send_broadcast_event.apply_async'
+    )
+    auth_header = create_service_authorization_header(service_id=sample_broadcast_service.id)
+
+    # create a broadcast
+    response_for_create = client.post(
+        path='/v2/broadcast',
+        data=sample_cap_xml_documents.WAINFLEET,
+        headers=[('Content-Type', 'application/cap+xml'), auth_header],
+    )
+    assert response_for_create.status_code == 201
+
+    response_json_for_create = json.loads(response_for_create.get_data(as_text=True))
+
+    broadcast_message = dao_get_broadcast_message_by_id_and_service_id(
+        response_json_for_create["id"], response_json_for_create["service_id"]
+    )
+    # approve broadcast
+    if is_approved:
+        broadcast_message.status = 'broadcasting'
+
+    first_response_for_cancel = client.post(
+        path='/v2/broadcast',
+        data=sample_cap_xml_documents.WAINFLEET_CANCEL_WITH_REFERENCES,
+        headers=[('Content-Type', 'application/cap+xml'), auth_header],
+    )
+    assert first_response_for_cancel.status_code == 201
+
+    second_response_for_cancel = client.post(
+        path='/v2/broadcast',
+        data=sample_cap_xml_documents.WAINFLEET_CANCEL_WITH_REFERENCES,
+        headers=[('Content-Type', 'application/cap+xml'), auth_header],
+    )
+    assert second_response_for_cancel.status_code == 404
+
+    assert len(mock_send_broadcast_event_task.call_args_list) == expected_cancel_tasks
+
+
 def test_large_polygon_is_simplified(
     client,
     sample_broadcast_service,