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Hello. Thanks for the question.
The rabies vaccine we use in dogs is the same vaccine that we use in cats. Essentially what you're asking is whether a double dose of the vaccine is harmful. It can induce a pretty significant immune response in the dog. This may lead to a fever, lethargy and potentially allergic reaction signs like vomiting, diarrhea, facial swelling and hives.
If needed, a short course of steroids (Prednisone) and Benadryl can help blunt the response. You can give Benadryl at home at a dose of 1-2 mg per lb body weight. It can be given every 6-8 hours, as needed.
I hope this helps, let me know if you have any other questions.