diff --git a/cmd/fmsgd/host.go b/cmd/fmsgd/host.go
index 194fed9..0235dbe 100644
--- a/cmd/fmsgd/host.go
+++ b/cmd/fmsgd/host.go
@@ -1396,6 +1396,14 @@ func downloadMessage(c net.Conn, r io.Reader, h *FMsgHeader, skipData bool) erro
 		return fmt.Errorf("%w actual hash: %s mismatch challenge response: %s", ErrProtocolViolation, actualHashStr, challengeHashStr)
 	}
 
+	// Duplicate detection keys on the message's canonical identity (msg.sha256),
+	// which for an add-to delivery is its original-form hash — not the add-to
+	// variant verified against the challenge above.
+	dupHash, err := canonicalMsgHash(h)
+	if err != nil {
+		return err
+	}
+
 	// pid/add-to validation is handled during header exchange in readHeader().
 
 	// determine file extension from MIME type
@@ -1423,7 +1431,7 @@ func downloadMessage(c net.Conn, r io.Reader, h *FMsgHeader, skipData bool) erro
 	acceptedAddTo := []FMsgAddress{}
 	var primaryFilepath string
 	for i, addr := range addrs {
-		code, err := validateMsgRecvForAddr(h, &addr, msgHash)
+		code, err := validateMsgRecvForAddr(h, &addr, dupHash)
 		if err != nil {
 			return err
 		}
@@ -1552,7 +1560,14 @@ func handleConn(c net.Conn) {
 	if header.ChallengeCompleted && header.InitialResponseCode != AcceptCodeAddTo {
 		addrs := localRecipients(header)
 		var err error
-		allLocalDup, err = allLocalRecipientsHaveMessageHash(header.ChallengeHash[:], addrs)
+		// Duplicate detection keys on msg.sha256 (the canonical original-form
+		// hash). For an add-to delivery that is header.Pid; the challenge hash
+		// is the add-to variant and would not match a stored row.
+		dupHash := header.ChallengeHash[:]
+		if header.Flags&FlagHasAddTo != 0 {
+			dupHash = header.Pid
+		}
+		allLocalDup, err = allLocalRecipientsHaveMessageHash(dupHash, addrs)
 		if err != nil {
 			log.Printf("ERROR: duplicate check failed for %s: %s", c.RemoteAddr().String(), err)
 			if err := sendCode(c, RejectCodeUndisclosed); err != nil {
diff --git a/cmd/fmsgd/sender.go b/cmd/fmsgd/sender.go
index fb30d3a..1f0a933 100644
--- a/cmd/fmsgd/sender.go
+++ b/cmd/fmsgd/sender.go
@@ -368,18 +368,11 @@ func deliverMessage(target pendingTarget) {
 
 	// Ensure sha256 is populated for this message so future pid lookups
 	// (e.g. add-to notifications or replies referencing it) can find it.
-	// For an add-to message the row being delivered IS the shared message,
-	// whose original-form hash loadMsg has already placed in h.Pid; calling
-	// h.GetMessageHash() here would instead hash the add-to variant.
 	var msgHash []byte
-	if h.Flags&FlagHasAddTo != 0 {
-		msgHash = h.Pid
-	} else {
-		msgHash, err = h.GetMessageHash()
-		if err != nil {
-			log.Printf("ERROR: sender: computing message hash for msg %d: %s", target.MsgID, err)
-			return
-		}
+	msgHash, err = canonicalMsgHash(h)
+	if err != nil {
+		log.Printf("ERROR: sender: computing message hash for msg %d: %s", target.MsgID, err)
+		return
 	}
 	if _, err := tx.Exec(`UPDATE msg SET sha256 = $1 WHERE id = $2 AND sha256 IS NULL`,
 		msgHash, target.MsgID); err != nil {
diff --git a/cmd/fmsgd/store.go b/cmd/fmsgd/store.go
index 825f7f1..830157d 100644
--- a/cmd/fmsgd/store.go
+++ b/cmd/fmsgd/store.go
@@ -176,6 +176,29 @@ func wirePidForLoadedMessage(storedParentHash []byte, msgHash []byte, hasAddTo b
 	return storedParentHash
 }
 
+// canonicalMsgHash returns the original-form message hash that is a message's
+// stable identity: it is stored in msg.sha256 and is what replies carry as
+// their wire pid. For an add-to message the row IS the shared message and its
+// original-form hash is already in msg.Pid (the add-to wire pid, SPEC §12);
+// GetMessageHash() there would instead hash the add-to variant and also needs
+// the message payload, which the code-11 path never downloads.
+func canonicalMsgHash(msg *FMsgHeader) ([]byte, error) {
+	if msg.Flags&FlagHasAddTo != 0 {
+		return msg.Pid, nil
+	}
+	return msg.GetMessageHash()
+}
+
+// relationalParentHash returns the hash of the message this one is a reply to,
+// or nil when there is none. An add-to message's Pid is its own identity, not
+// a parent pointer (SPEC §12), so it must never be resolved as a parent.
+func relationalParentHash(msg *FMsgHeader) []byte {
+	if msg.Flags&FlagHasAddTo != 0 {
+		return nil
+	}
+	return msg.Pid
+}
+
 // getMsgByID loads a message and all its recipients from the database by msg ID.
 // Returns the full FMsgHeader or nil if the message doesn't exist.
 func getMsgByID(msgID int64) (*FMsgHeader, error) {
@@ -204,6 +227,63 @@ func getMsgByID(msgID int64) (*FMsgHeader, error) {
 	return h, nil
 }
 
+// existingMsgIDForAddTo returns the id of an already-stored message row whose
+// canonical sha256 matches msgHash, for an add-to delivery. It returns 0 when
+// the message is not an add-to message or no such row exists, so the caller
+// falls through to a normal INSERT. Non-add-to messages never take the attach
+// path: a colliding sha256 there is a genuine duplicate, not a shared message.
+func existingMsgIDForAddTo(tx *sql.Tx, msg *FMsgHeader, msgHash []byte) (int64, error) {
+	if msg.Flags&FlagHasAddTo == 0 || len(msgHash) == 0 {
+		return 0, nil
+	}
+	var id int64
+	err := tx.QueryRow("SELECT id FROM msg WHERE sha256 = $1", msgHash).Scan(&id)
+	if err == sql.ErrNoRows {
+		return 0, nil
+	}
+	return id, err
+}
+
+// attachAddToRecipients extends an already-stored message with the recipients
+// carried by an add-to delivery, marking those on our domain delivered. It is
+// the add-to counterpart of an INSERT: when a host already holds the shared
+// message, an add-to delivery must grow its recipient list rather than insert
+// a second row under the same unique canonical sha256 (SPEC §12).
+func attachAddToRecipients(tx *sql.Tx, msgID int64, msg *FMsgHeader) error {
+	now := timeutil.TimestampNow().Float64()
+
+	add := func(table string, addr FMsgAddress) error {
+		var delivered interface{}
+		if addr.Domain == Domain {
+			delivered = now
+		}
+		_, err := tx.Exec(`insert into `+table+` (msg_id, addr, time_delivered)
+values ($1, $2, $3)
+on conflict (msg_id, addr) do nothing`, msgID, addr.ToString(), delivered)
+		return err
+	}
+
+	for _, addr := range msg.To {
+		if err := add("msg_to", addr); err != nil {
+			return err
+		}
+	}
+	for _, addr := range msg.AddTo {
+		if err := add("msg_add_to", addr); err != nil {
+			return err
+		}
+	}
+
+	if msg.AddToFrom != nil {
+		if _, err := tx.Exec(`update msg set add_to_from = $1
+where id = $2 and (add_to_from is null or add_to_from = '')`,
+			msg.AddToFrom.ToString(), msgID); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
 func storeMsgDetail(msg *FMsgHeader) error {
 
 	db, err := sql.Open("postgres", "")
@@ -218,10 +298,23 @@ func storeMsgDetail(msg *FMsgHeader) error {
 	}
 	defer tx.Rollback()
 
-	msgHash, err := msg.GetMessageHash()
+	msgHash, err := canonicalMsgHash(msg)
 	if err != nil {
 		return err
 	}
+	parentHash := relationalParentHash(msg)
+
+	// An add-to delivery for a message this host already holds extends the
+	// existing row's recipient list; inserting again would collide on the
+	// unique canonical sha256 (SPEC §12).
+	if existingID, err := existingMsgIDForAddTo(tx, msg, msgHash); err != nil {
+		return err
+	} else if existingID != 0 {
+		if err := attachAddToRecipients(tx, existingID, msg); err != nil {
+			return err
+		}
+		return tx.Commit()
+	}
 
 	var addToFrom interface{}
 	if msg.AddToFrom != nil {
@@ -254,7 +347,7 @@ returning id`,
 		msg.Topic,
 		msg.Type,
 		msgHash,
-		msg.Pid,
+		parentHash,
 		int(msg.Size),
 		msg.Filepath).Scan(&msgID)
 	if err != nil {
@@ -316,7 +409,7 @@ values ($1, $2, $3, $4, $5, $6, $7)`)
 		}
 	}
 
-	if err := resolveMsgParentLinks(tx, msgID, msgHash, msg.Pid, requiresStoredParent(msg)); err != nil {
+	if err := resolveMsgParentLinks(tx, msgID, msgHash, parentHash, requiresStoredParent(msg)); err != nil {
 		return err
 	}
 
@@ -340,10 +433,23 @@ func storeMsgHeaderOnly(msg *FMsgHeader) error {
 	}
 	defer tx.Rollback()
 
-	msgHash, err := msg.GetMessageHash()
+	msgHash, err := canonicalMsgHash(msg)
 	if err != nil {
 		return err
 	}
+	parentHash := relationalParentHash(msg)
+
+	// An add-to delivery for a message this host already holds extends the
+	// existing row's recipient list; inserting again would collide on the
+	// unique canonical sha256 (SPEC §12).
+	if existingID, err := existingMsgIDForAddTo(tx, msg, msgHash); err != nil {
+		return err
+	} else if existingID != 0 {
+		if err := attachAddToRecipients(tx, existingID, msg); err != nil {
+			return err
+		}
+		return tx.Commit()
+	}
 
 	var addToFrom interface{}
 	if msg.AddToFrom != nil {
@@ -376,7 +482,7 @@ returning id`,
 		msg.Topic,
 		msg.Type,
 		msgHash,
-		msg.Pid,
+		parentHash,
 		int(msg.Size),
 		"").Scan(&msgID)
 	if err != nil {
@@ -425,7 +531,7 @@ values ($1, $2, $3, $4, $5, $6, $7)`)
 		}
 	}
 
-	if err := resolveMsgParentLinks(tx, msgID, msgHash, msg.Pid, requiresStoredParent(msg)); err != nil {
+	if err := resolveMsgParentLinks(tx, msgID, msgHash, parentHash, requiresStoredParent(msg)); err != nil {
 		return err
 	}
 
diff --git a/cmd/fmsgd/store_test.go b/cmd/fmsgd/store_test.go
index 6bfdf98..98a5687 100644
--- a/cmd/fmsgd/store_test.go
+++ b/cmd/fmsgd/store_test.go
@@ -137,3 +137,47 @@ func TestWirePidForLoadedMessageReplyKeepsParentHash(t *testing.T) {
 		t.Fatalf("reply wire pid = %v, want parent hash %v", got, parentHash)
 	}
 }
+
+// An add-to message's row IS the shared message; its canonical hash is the
+// original-form hash carried in Pid, not the add-to variant. Storing the
+// variant would make replies reference a hash the origin host never knows.
+func TestCanonicalMsgHashAddToUsesPid(t *testing.T) {
+	origHash := []byte{9, 8, 7, 6}
+
+	got, err := canonicalMsgHash(&FMsgHeader{Flags: FlagHasPid | FlagHasAddTo, Pid: origHash})
+	if err != nil {
+		t.Fatalf("canonicalMsgHash returned error: %v", err)
+	}
+	if !bytes.Equal(got, origHash) {
+		t.Fatalf("add-to canonical hash = %v, want original-form hash %v", got, origHash)
+	}
+}
+
+// A non-add-to message must never take the add-to attach path: a colliding
+// sha256 there is a genuine duplicate, not the shared message being extended.
+// existingMsgIDForAddTo short-circuits before touching the database for it.
+func TestExistingMsgIDForAddToSkipsNonAddTo(t *testing.T) {
+	id, err := existingMsgIDForAddTo(nil, &FMsgHeader{Flags: FlagHasPid}, []byte{1, 2, 3})
+	if err != nil {
+		t.Fatalf("existingMsgIDForAddTo returned error: %v", err)
+	}
+	if id != 0 {
+		t.Fatalf("non-add-to message returned existing id %d, want 0", id)
+	}
+}
+
+// An add-to message's Pid identifies the message itself, not a parent, so it
+// must not be resolved as a relational parent. A plain reply's Pid is a parent.
+func TestRelationalParentHashAddToHasNoParent(t *testing.T) {
+	pid := []byte{1, 2, 3}
+
+	if got := relationalParentHash(&FMsgHeader{Flags: FlagHasPid | FlagHasAddTo, Pid: pid}); got != nil {
+		t.Fatalf("add-to relational parent = %v, want nil", got)
+	}
+	if got := relationalParentHash(&FMsgHeader{Flags: FlagHasPid, Pid: pid}); !bytes.Equal(got, pid) {
+		t.Fatalf("reply relational parent = %v, want %v", got, pid)
+	}
+	if got := relationalParentHash(&FMsgHeader{}); got != nil {
+		t.Fatalf("new-thread relational parent = %v, want nil", got)
+	}
+}
diff --git a/dd.sql b/dd.sql
index ebc6507..6d4c07c 100644
--- a/dd.sql
+++ b/dd.sql
@@ -132,11 +132,26 @@ create trigger trg_msg_prevent_unreferenceable_parent
     before update of time_sent, sha256 on msg
     for each row execute function prevent_referenced_msg_from_becoming_unreferenceable();
 
--- notify when a new msg_to row is inserted with null time_delivered so the
--- sender can pick it up immediately instead of waiting for the next poll.
-create or replace function notify_msg_to_insert() returns trigger as $$
+-- Notify the sender's outgoing worker (channel new_msg_to) whenever new
+-- delivery work appears. One function serves all three triggers, dispatching
+-- on the table it fired for:
+--   * msg               -- a draft message transitions to sent (time_sent set
+--                          for the first time); notify every recipient.
+--   * msg_to/msg_add_to -- a recipient row is inserted against an already-sent
+--                          message (recipients added via add-to after the
+--                          message was sent); notify that recipient.
+-- The payload is advisory only: the worker re-polls fully on any wake-up.
+create or replace function notify_msg_sent() returns trigger as $$
 begin
-    if NEW.time_delivered is null then
+    if TG_TABLE_NAME = 'msg' then
+        if OLD.time_sent is null and NEW.time_sent is not null then
+            perform pg_notify('new_msg_to', NEW.id::text || ',' || addr)
+            from msg_to where msg_id = NEW.id;
+
+            perform pg_notify('new_msg_to', NEW.id::text || ',' || addr)
+            from msg_add_to where msg_id = NEW.id;
+        end if;
+    elsif NEW.time_delivered is null then
         perform pg_notify('new_msg_to', NEW.msg_id::text || ',' || NEW.addr)
         from msg where id = NEW.msg_id and time_sent is not null;
     end if;
@@ -147,11 +162,15 @@ $$ language plpgsql;
 drop trigger if exists trg_msg_to_insert on msg_to;
 create trigger trg_msg_to_insert
     after insert on msg_to
-    for each row execute function notify_msg_to_insert();
+    for each row execute function notify_msg_sent();
 
--- notify when a new msg_add_to row is inserted with null time_delivered so the
--- sender can pick it up immediately instead of waiting for the next poll.
 drop trigger if exists trg_msg_add_to_insert on msg_add_to;
 create trigger trg_msg_add_to_insert
     after insert on msg_add_to
-    for each row execute function notify_msg_to_insert();
+    for each row execute function notify_msg_sent();
+
+drop trigger if exists trg_msg_sent on msg;
+create trigger trg_msg_sent
+    after update on msg
+    for each row execute function notify_msg_sent();
+