Unique insert_id
Event sent
insert_id: a1
Retry (network)
insert_id: a1
Amplitude stores 1 event
duplicate ignored
Correct: safe retry
Reused insert_id
Purchase #1
(insert_id: x9)
Purchase #2
(insert_id: x9)
Amplitude drops the second
real event lost
Same device_id + insert_id
within seven days.