Q. If when davening one looses concentration and says some tefilos, brochos and mizmorim without Kavanah, can one repeat them?
A. Horav Shlomo Miller’s Shlit’a opinion is that on the Mizmorim, psalms and Bible chapters recited in Pesukei Dezimra one can repeat them when said without intention as long as he says a complete Pasuk.
During the blessings of Shema or Shemone Ezrei one should not repeat words that were said complete even when not recited with Kavana.
The chapters of Shema can be repeated after the fact, if said without intention.
Rabbi A. Bartfeld as revised by Horav Dovid Pam, Horav Aharon Miller and Horav Chanoch Ehrentreu Shlit’a