At this level you should know this will happen and have placeholders in place.
If every single IP is unique, additional layer would be pattern matching traffic by different network classes and flagging it actively.
Another way to rate limit is by number of pages per session / time.
Then you can have another fail safe method where if X exceeds Y start throttling top X (dynamically scale up until X is returned to normal) while notifying DevOps of the issue.
Etc.