The medium/large size (the classic size) always did have the double flap only, unless it was a rare or special model of the flap bag. I believe up until 2010, the jumbos and maxis only came in single flaps, and from 2010 and afterwards, the jumbos and maxis started coming out in double flaps only. I didn't start buying Chanel until this year (though I would have if I could have years and years ago), but from what I have read on here, it seems that the quality started going down around the same time they stopped making bags with gold-plated hardware and single flaps.
I much prefer the double flap as well. It is the way Coco Chanel originally designed it, and I love the compartmentalizations of the bag with the double flap, rather than the simple bucket-like interiors of the single flaps (though they are still beautiful). The double flap also holds the structure of the bag better.
Either way, I'm sure you will be happy with the bag
In part because Chanel may have started using inferior caviar and stopped 18k or 22k gold plating the CC, they can justify double-flapping the jumbo and maxis; it is afterall added material and labor costs to add the flap. My 2008 single flap jumbo is sturdy and rigid no matter how much weight I carry.
I have not bought new since 2008 but all the news tells me that Chanel is quite dissappointing lately.
The m/l has always been a double flap except for seasonal pieces while the jumbo flaps are now all double flaps since 2011. I prefer the double flap since it holds the shape of the bag better but it can get really heavy . The jumbo single flap is lighter and can fit more.