I accidentally took two Claritin D about one hour apart. This is the 12-hour formula. Is there a problem with this?

Unlikely. You've had a double dose of the decongestant which can keep you from sleeping well, increase your heart rate and increase your blood pressure. The double dose of Claritin is unlikely to cause you any issues in the next 12 hours. I do not think you need any immediate medical attention unless you develop any concerning symptoms.