Apache Storm Component Guide
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Understanding the at-least-once Guarantee

The windowing functionality in Storm core provides an at-least-once guarantee. Values emitted from a bolt’s execute(TupleWindow inputWindow) method are automatically anchored to all tuples in inputWindow. Downstream bolts are expected to acknowledge the received tuple (the tuple emitted from the windowed bolt) to complete the tuple tree. If not acknowledged, the tuples are replayed and the windowing computation is reevaluated.

Tuples in a window are automatically acknowledged when they exit the window after windowLength + slidingInterval. Note that the configuration topology.message.timeout.secs should be more than windowLength + slidingInterval for time-based windows; otherwise, the tuples expire and are replayed, which can result in duplicate evaluations. For count-based windows, you should adjust the configuration so that windowLength + slidingInterval tuples can be received within the timeout period.