  Relativistic shocks provide an efficient method for high-energy particle
acceleration in many astrophysical sources. Multiple shock systems are even
more effective and of importance, for example, in the internal shock model of
gamma-ray bursts. We investigate the reacceleration of pre-existing energetic
particles at such relativistic internal shocks by the first order Fermi process
of pitch angle scattering. We use a well established eigenfunction method to
calculate the resulting spectra for infinitely thin shocks. Implications for
GRBs and relativistic jets are discussed.
