The detection of damage associated shock pulses via the deflection of a probe laser beam is exploited to study multiple-shot damage thresholds in CaF², PMMA, copper, and aluminium. We find a reduction of threshold intensity as compared to single-shot conditions. The experimental results point towards deviations from the previously postulated fluence dependence F^N = F¹N^S-1 [see Ref.4]. They are, however, compatible with a simple model relating the threshold reduction to defect accumulation via non-linear energy absorption.