Hello, my name is***** and I have over 20 years of experience as a veterinarian. I understand that you are concerned that you accidentally your pup two Sentinel flavor tablets rather than the one normally given for flea and heartworm protection.
You don't say how much he weighs, but this is likely to be fine.
This heartworm/flea prevention contains milbemycin oxime and lufeneron.
The good news is that both of these drugs are very safe and very well tolerated in dogs.
In fact we can use much higher doses of milbemycin then we use for heartworm prevention to treat demodectic mange, up to four to five times the dose used for heartworm protection. Beagles were given a one time dose of 90mgs of milbemycin per pound of body weight in an experimental setting with no adverse effects.
Lufenuron only affects insects in that it interferes with chitin synthesis, which is an integral part of their egg production. Because mammals don't produce chitin, it's not an issue for them.
There have been reports of nausea and diarrhea in some dogs taking Sentinel, even at normal doses, so you may see that with your pups, but that should be all that you see. I wouldn't worry about a one time high dose of this medication.
Please let me know if you have any further questions