Can U-238 undergo regular, single beta decay?
Are there isotopes which can only undergo double, never single, beta decay? Not even two normal beta decays in quick succession?
The U238 (and some other isotopes) can't decay with single beta, but only with double beta, because of energetic reasons. The decay product of single beta would have higher energy than the energy of the original nucleus, but with double beta they can decay into a lower energy state. (Look at the isotope masses of U238, Np238 and Pu238, higher mass means higher energy.)