Fix typo in comment ("if if")

docs/tutorials/task-cookbook.rst

@@ -45,7 +45,7 @@ The cache key expires after some time in case something unexpected happens
         feed_url_digest = md5(feed_url).hexdigest()
         lock_id = '{0}-lock-{1}'.format(self.name, feed_url_hexdigest)
 
-        # cache.add fails if if the key already exists
+        # cache.add fails if the key already exists
         acquire_lock = lambda: cache.add(lock_id, 'true', LOCK_EXPIRE)
         # memcache delete is very slow, but we have to use it to take
         # advantage of using add() for atomic locking