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Although most nuts should be avoided by dogs, peanuts are not truly a nut and don't cause the problems that true nuts can. I am attaching a link for you that should help to calm your worries, but if your dog has any problems such as excessive vomiting and/or diarrhea that doesn't stop, or if he becomes lethargic or doesn't act 'right', you should go ahead and get him into the vet so he can be checked out and monitored. Otherwise, since this only happened the one time, I wouldn't worry too much.
Please let me know if I can help further.