Multiple Exposures
Source Player Core pg. 422
Multiple exposures to the same curse or disease currently affecting you have no effect. For a poison, however, failing the initial saving throw against a new exposure increases the stage by 1 (or by 2 if you critically fail) without affecting the maximum duration. This is true even if you're within the poison's onset period, though it doesn't change the onset length. If the poison doesn't have an onset time or it's already elapsed, you're immediately subject to the effects of the new stage.