I accidently took two allegra (fexofenadine) today, one this am, one just now. Should I be worried?

No. Allegra (fexofenadine) has been extensively studied for side effects; none were apparent at up to 5 times the recommended dosage. I wouldn't increase the dosage on a regular basis unless it's under an allergist's supervision, but i wouldn't worry about an occasional double dose.
Be happy. You did not mention what strength of Allegra (fexofenadine) you took, but i assume you are worried because you took 2 tablets of 180mg each. We use this dose quite often for patients with severe rashes or hives. Also, during the clinical studies prior to the release of allegra, (fexofenadine) doses of up to 540mg were used without ill effects.