Can i take allegra (fexofenadine) d and wall-som sleep aid at the same time.

Drug duplication: I assume you're referring to Wal-Som brand of diphenhydramine (https://www.drugs.com/otc/119159/wal-som.html) which is often used for its sedative properties. In actuality, diphenhydramine is an antihistamine in same class as fexofenadine, active ingredient of Allegra (and Allegra-D) so there's duplication of drugs and potentially their effects. Check out https://goo.gl/T9ytK3 for interactions