As with every Reccuring Quest it is possible to win 20 diamonds (Chateau Fronteac bonus applies) only once.
OBVIOUSLY if you could win diamonds many times that would be bad business for Inno in an age like BA, HMA, CA, etc. My 38 blacksmith city would be pooping out diamonds every single day LOL with less effort than diamonds farming!
Diamond farming would have a whole new look with 20 diamonds every BA city, X that by 23 worlds and the fact that Reccuring Quest Looping is already a pretty good strategy and you're making 460 free diamonds per day, which would not be good for Inno Business. Then get all those cities into CA/HMA and you have disaster mode
Story Telling Time, Everyone!
Here's a fun fact... back a few years ago when Forge of Empires was in complete beta test mode, you
COULD win diamonds unlimited times, or so I've heard. A player documented this strategy and made thousands of diamonds using this strategy
I'm not kidding. So Inno saw it and obviously made diamonds a one-time reward.