一桩桩 is a reduplicated measure word pattern meaning 'one by one' or 'a series of' — specifically for matters, events, or cases (the classifier 桩 is used for items like legal cases, transactions, or incidents). The reduplication emphasizes that each event is distinct and countable, often with a cumulative or enumerative feel. You'll hear it in contexts like listing grievances or recounting a sequence of events: 一桩桩一件件 ('every single matter and item').