  We report the observation of a new structure in the $\Lambda_b^0 \pi^+\pi^-$
spectrum using the full LHCb data set of $pp$ collisions, corresponding to an
integrated luminosity of $9\,\mathrm{fb}^{-1}$, collected at $\sqrt{s}=7$, 8
and $13\,\mathrm{TeV}$. A study of the structure suggests its interpretation as
a superposition of two almost degenerate narrow states. The masses and widths
of these states are measured to be
  \begin{eqnarray*}
  m_{\Lambda_{b}\mathrm{(6146)}^0} &=&6146.17 \pm 0.33 \pm 0.22 \pm
0.16\,\mathrm{MeV} \,,
  m_{\Lambda_{b}\mathrm{(6152)}^0} &=&6152.51 \pm 0.26 \pm 0.22 \pm
0.16\,\mathrm{MeV} \,,
  \Gamma_{\Lambda_{b}\mathrm{(6146)}^0}&=&\phantom{615}2.9\phantom{0}\pm
1.3\phantom{0} \pm 0.3\phantom{0}\,\mathrm{MeV}\,,
  \Gamma_{\Lambda_{b}\mathrm{(6152)}^0} &=&\phantom{615}2.1\phantom{0}\pm
0.8\phantom{0} \pm 0.3\phantom{0}\,\mathrm{MeV}\,,
  \end{eqnarray*} with a mass splitting of $\Delta m = 6.34 \pm 0.32 \pm
0.02\,\mathrm{MeV}$, where the first uncertainty is statistical, the second
systematic and the third derives from the knowledge of the mass of the
$\Lambda_b^0$ baryon. The measured masses and widths of these new excited
states suggest their possible interpretation as a doublet of
$\Lambda_{b}\mathrm{(1D)}^0$ states.
