Atenolol slows your heart rate and decreases your blood pressure. Your dose is not a high one, but people can respond individually so doubling it accidentally can make you sick. Depending on why you are taking the medication, it may be more or less advisable to take another tablet if you are unsure of whether you took it. I would normally instruct my patients not to take a second dose unless they are certain that their pulse is fast and blood pressure high. I would try and relax and perhaps see a nurse/healthcare provider to measure vital signs and help you to make a more informed decision in this case.