docs(receiving-messages): PDO's once-per-message gate is per sender

guides/receiving-messages.mdx

@@ -620,6 +620,10 @@ PDO is also used alongside retry receipts for normal decryption failures. On the
 PDO requests are deduplicated — if a request is already pending for a given message, subsequent requests are skipped. Pending requests expire after 30 seconds. The deduplication cache uses phone-number JIDs as keys (not LID JIDs) to ensure the cache key matches the JID format in the phone's response.
 </Note>
 
+Separately from that 30-second in-flight cache, a longer-lived `pdo_requested` memo cache also gates placeholder requests, capping each message to at most one request within its window (default: 24h TTL, 512 entries — see [Bot - Cache Configuration Reference](/api/bot#cache-configuration-reference)). Expiration or eviction can still let a later redelivery send a second request for the same message.
+
+**This gate now keys per sender, not just per `(chat, id)` ([#1319](https://github.com/oxidezap/whatsapp-rust/pull/1319)).** The sending client chooses a message's ID, not the server, so two participants in the same group can pick the same one. Before this fix, gating on `(chat, id)` alone let the first participant's request silently suppress the second's, and the second sender's content was never recovered. When a PDO response arrives, the client also checks that the pending request it matches names the same sender, comparing both the LID and phone-number spellings since a group's own delivery can carry either. If the sender doesn't match, the client rebuilds the recovered message from the response itself instead of risking a wrong attribution.
+
 ### Sent message retry (outbound)
 
 When a recipient's device cannot decrypt your message, it sends a retry receipt. The client handles this automatically using DB-backed sent message storage: