There is a lot of talk surrounding whether or not you should shave a double coated dog. The information being shared here is based off of my own personal experience. I am not recommending that anyone shave their double coated dog if it isn't necessary, just sharing the information I gathered.
I shaved my double coated dog LiLo in July of this year - here she is before clipping:
Here she is after clipping in July:
I used an 8.5 blade so it wasn't super close to the skin, but it completely exposed her undercoat. Even at this length what was left of the undercoat protected her skin and when I bathed her it tried to repel the water.
It has taken 5mos for LiLo's hair coat to return to almost normal, this picture was taken earlier this month:
LiLo is 10 years old and has no health issues other than hip dysplasia.
It will be 5mos in December since I clipped my double coated dog. Her hair coat [although it is hard to see in the pictures] is not completely grown in. I shared pictures of LiLo in a group on facebook and had multiple groomers tell me that every time they shaved a double coated dog, the hair never grew back correctly or was fuzzy. Most people who actually have their dogs shaved regularly will bring them in every 8-12 weeks, that is NOT enough time for the hair coat to recover, so of course the groomer will see a fuzzy coat.
Here is a picture of LiLo mid hair growth in August:
And here she is in September:
So you can see, if a person was on a regular grooming schedule, if they brought the dog back within an 8-12 week period the hair coat would be in a weird state of growth.
Hormones also play a huge role when it comes to the hair coat. Here are pictures of a Samoyed before and after her spay. I wish I could give credit to the dog's owner, I saw these pictures in an ovary sparing spay group, but if you google "samoyed before and after spay" these pictures come up on reddit:
If a groomer has long time clients - from puppy to adult, they might see these kinds of hair coat changes and it may have nothing to do with being shaved, it may be the difference between the dog being spayed or neutered.
Shaving the hair coat closely and often, with something like a 40 blade can damage the hair follicles which would cause the hair to not regrow, however I don't know many people who shave their dogs with a 40 blade.
That being said, a 40 or 50 blade is used to prep dogs for surgery, and I know many double coated dogs who have had knees surgeries, the whole leg has to be shaved and while it takes time [6mos+] the hair always grows back. Unless a dog has a health issue, like a thyroid problem, the hair should grow back.
I have heard a lot of stories about ruining the double coat by shaving but I have yet to see a scientific study or any true research that is not a personal account or opinion. This post is not a scientific study, as I mentioned above this is simply to share my experience. I shaved my dog because I wanted to see if it would grow back okay, and I also thought clipping her like a lion would be cute. I don't recommend shaving your dog for no reason, but I wouldn't condemn a person in a hot climate wanting to help cool their dog, or if you own a dog who will not let you actually brush the under coat out - a shaved dog is better than a matted one. If you do not take care of the under coat by brushing it out, your dog's coat will be completely ineffective in any temperature, it is also impossible to bathe your dog properly without first brushing out the loose undercoat. If you do not maintain the undercoat through regular brushing the hair will mat and be painful to remove, it will also make it impossible for you to get the hair coat fully clean.
Anyway, in closing, I think if a double coated dog is healthy, you can shave it and the hair coat should recover, but it needs at least 6 months to a year to go back to completely normal.