diff --git a/docs/telemetry-contract.md b/docs/telemetry-contract.md
index ff285d8..4220c87 100644
--- a/docs/telemetry-contract.md
+++ b/docs/telemetry-contract.md
@@ -30,6 +30,14 @@ older than 14 days nightly.
 `host` is only the stream's `scheme://host` — **never** a full signed URL or an
 addon token (redacted client-side in `TelemetryService.redactUrl` before write).
 
+**Rows from builds up to 1.3.0+38 can be stored twice.** Sending the app to the
+background started two flushes at once (`onHide` and `onPause`), and both posted
+the pending rows. Measured 2026-08-30 to 09-13: 17 of 438 sessions have two
+`session_summary` rows, on six devices, each pair created within 0.23 s, and every
+other kind is affected the same way. Count distinct `device` + `ts` + `event` (for
+a summary, `device` + `meta.sessionId`), not rows. Builds after 1.3.0+38 run one
+flush at a time.
+
 ### `source_pick` — the row that answers "why did it play that one"
 
 Added 2026-09-09, because a real complaint could not be answered without it.