  We consider fluid dynamics of relativistic double explosion - when a point
explosion with energy $E_1$ is followed by a second explosion with energy $E_2$
after time $t_d$ (the second explosion could be in a form of a long lasting
wind). The primary explosion creates a self-similar relativistic blast wave
propagating with \Lf\ $\Gamma_1(t)$. A sufficiently strong second explosion,
with total energy $E_2 \geq 10^{-2} E_1$, creates a fast second shock in the
external fluid previously shocked by the primary shock. At times longer than
the interval between the explosions $t_d$, yet short compared with the time
when the second shock catches up the primary shock at $\sim t_d \Gamma_1^2$,
the structure of the second shock is approximately self-similar. Self-similar
structure of the second shock exist for the case of constant external density
(in this case $\Gamma_2 \propto t^{-7/3}$), but not for the wind environment.
  At early times the \Lf\ of the second shock may exceed that of the primary
shock and may boost the synchrotron emission of locally accelerated electrons
into the Fermi LAT range.
