Q.   If someone recites Tasbeeh in Ruku and Sajdah only once, will it be valid?

A.   Something could be valid but sinful. An act of ibaadat could be valid but sinfully discharged. Reciting the Tasbeeh only once in Sajdah is in conflict with the Sunnah. If one adopts this as a permanent practice without valid reason, then whilst the  Salaat will be valid, one will be sinful for deliberately violating the Sunnah without valid reason.