Activity ID conflict policy: default vs USE_EXISTING

Two upstream calls fire start_activity(id="evt_001"). Server decides what happens to the second one.
Default policy
id_conflict_policy=FAIL (the default)
Caller A
start_activity(id="evt_001")
 
Caller B
start_activity(id="evt_001")
 
Temporal server
Standalone Activities registry
evt_001: Caller A scheduled ✓
evt_001: Caller B rejected ✗ already started
Worker
idle
Activities started
0
Calls that errored
0
With USE_EXISTING
id_conflict_policy=USE_EXISTING
Caller A
start_activity(id="evt_001")
 
Caller B
start_activity(id="evt_001")
 
Temporal server
Standalone Activities registry
evt_001: Caller A scheduled ✓
evt_001: Caller B returned existing handle ✓
Worker
idle
Activities started
0
Calls that errored
0
Same Worker in both lanes. The server dedupes by Activity ID before the duplicate ever reaches the Task Queue, so the Worker only runs one Activity either way. The policy decides what Caller B hears back.