I doubt it if you understand what a double dewclaw is. It is major surgery however if the dog has an extra toe on the hind leg which is fully formed.
Suppose you have a dog like a Great Pyrenees. There is a bone structure muscles and nerves in the rear dewclaws. Dewclaw removal implies cutting of bones and severing of nervous connections.
Even though the surgery is performed at a young age the puppy is bound to recover quickly but it is not known how the surgery will affect its development in the future.
Dogs of large breeds grow up more slowly than dogs of small breeds. At 18-24 months of age the Great Pyrenees or Anatolian shepherd will reach maturity.
By removing a toe that was meant to provide support you might end up with balance problems in the future.
Accidents involving the extra digit in the rear are rare in dogs with functional double dewclaws. A dogs toe is an integral part of the leg and it learns how to deal with it from an early age.