What happens if i accidentally take 2 claritin (loratadine) d in one day?

It depends: There are two different doses of Claritin (loratadine) d - a twelve hour dose and a twenty four hour dose. Obviously, the 12 hour dose is recommended twice a day, 12 hours apart. If you were to take two 24 hour tablets, you would be getting the equivalent of 16 sudafed tablets. That could cause agitation, insomnia, rapid heart rate, and an elevation in blood pressure. That would not be recommended.