The record
In a thread started December 27, 2016, a user says double tapping either of their new AirPods nearly always failed to bring up Siri on their iPhone 7 with iOS 10.2. By their own estimate it worked only twice, both when they tapped the side of an earbud. They later said checking the double tap settings, as suggested in the thread, did not fix it, and no fix from the original poster appears before the thread closed. Other participants described different workarounds that helped them, but this entry reflects one user's experience only.
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A documented user report, not an independently reproduced test or an estimate of how often the problem occurs.