  The double pion photoproduction off nucleons near threshold is analyzed in a
covariant baryon chiral perturbation theory up to next to leading order, where
the $\Delta(1232)$, $N^*(1400)$ and $\rho(770)$ resonances are included as
explicit degrees of freedom. For the process $\gamma p \to \pi^+ \pi^0 n$, the
chiral results of total cross sections, invariant-mass distributions and
beam-helicity asymmetry are in good agreement with the experimental data within
uncertainties. For the process $\gamma p \to \pi^0 \pi^0 p$, the prediction of
total cross section deviates from the existing experimental data. Once the
final-state interaction of $\pi \pi$ in the isoscalar S-wave channel is taken
into account, a good description of the cross section is achieved. The effect
of the Roper resonance always turns out be negligible, and hence can be thrown
away in future study of this process.
