Depende do tipo de aflição - para doenças e xingamentos, múltiplas exposições simplesmente dão múltiplas chances de contrair a aflição. Para venenos, os efeitos se acumulam, mas possivelmente não do jeito que você espera.
Na subseção Poison da seção Afflictions do Glossário -
Unlike other afflictions, multiple doses of the same poison stack. Poisons delivered by injury and contact cannot inflict more than one dose of poison at a time, but inhaled and ingested poisons can inflict multiple doses at once. Each additional dose extends the total duration of the poison (as noted under frequency) by half its total duration. In addition, each dose of poison increases the DC to resist the poison by +2. This increase is cumulative. Multiple doses do not alter the cure conditions of the poison, and meeting these conditions ends the affliction for all the doses. For example, a character is bit three times in the same round by a trio of Medium monstrous spiders, injecting him with three doses of Medium spider venom. The unfortunate character must make a DC 18 Fortitude save for the next 8 rounds. Fortunately, just one successful save cures the character of all three doses of the poison.