Hide a reference number in the form. Record it server-side when the form is first submitted.

When the results are available, store them with the reference number.

If you get a request you've already dealt with, you can serve the results immediately, from your cache.

Any use to you?

Andy.

update: I re-read the question. I guess you'd only need to keep a list of 'used' reference numbers, not each actual results set.