I too would be very interested to hear about someone actually doing a Diamond Challenge twice. I'm pretty confident of re-qualifying this year, and will do my best next year as well, but with only leisure travel, I doubt I can keep it up indefinitely, and to know that I can re-challenge again some time in the future if necessary would make things less stressful.
I heard the "One trial offer per member" statement explained as meaning one Challenge per trial period (i.e. once in the first half of the year, but possible to try again in the second half); no idea if that is true.
On the other hand, GP T&C are often so poorly written as to be open to a wide range of interpretation.