diff --git a/internal/db/queries.go b/internal/db/queries.go
index be27ff3..655fa04 100644
--- a/internal/db/queries.go
+++ b/internal/db/queries.go
@@ -1783,45 +1783,33 @@ func (database *Database) SetChannelHashcashBits(
 }
 
 // RecordSpentHashcash stores a spent hashcash stamp hash
-// for replay prevention.
+// for replay prevention. It returns false when the hash was
+// already stored. The one INSERT is both the check and the
+// record, so of several concurrent callers with the same
+// hash exactly one gets true.
 func (database *Database) RecordSpentHashcash(
 	ctx context.Context,
 	stampHash string,
-) error {
-	_, err := database.conn.ExecContext(ctx,
+) (bool, error) {
+	res, err := database.conn.ExecContext(ctx,
 		`INSERT OR IGNORE INTO spent_hashcash
 		 (stamp_hash, created_at)
 		 VALUES (?, ?)`,
 		stampHash, time.Now())
 	if err != nil {
-		return fmt.Errorf(
+		return false, fmt.Errorf(
 			"record spent hashcash: %w", err,
 		)
 	}
 
-	return nil
-}
-
-// IsHashcashSpent checks whether a hashcash stamp hash
-// has already been used.
-func (database *Database) IsHashcashSpent(
-	ctx context.Context,
-	stampHash string,
-) (bool, error) {
-	var count int
-
-	err := database.conn.QueryRowContext(ctx,
-		`SELECT COUNT(*) FROM spent_hashcash
-		 WHERE stamp_hash = ?`,
-		stampHash,
-	).Scan(&count)
+	inserted, err := res.RowsAffected()
 	if err != nil {
 		return false, fmt.Errorf(
-			"check spent hashcash: %w", err,
+			"record spent hashcash: %w", err,
 		)
 	}
 
-	return count > 0, nil
+	return inserted > 0, nil
 }
 
 // PruneSpentHashcash deletes spent hashcash tokens older
diff --git a/internal/handlers/api_test.go b/internal/handlers/api_test.go
index 31861f3..fca94c9 100644
--- a/internal/handlers/api_test.go
+++ b/internal/handlers/api_test.go
@@ -3368,6 +3368,137 @@ func TestChannelHashcashRejectReplayedStamp(t *testing.T) {
 	}
 }
 
+// sendConcurrently posts cmd in the given number of requests
+// at once and returns the status field of each response.
+func (tserver *testServer) sendConcurrently(
+	token string, cmd map[string]any, requests int,
+) []any {
+	tserver.t.Helper()
+
+	body, err := json.Marshal(cmd)
+	if err != nil {
+		tserver.t.Fatalf("marshal command: %v", err)
+	}
+
+	// Each request writes only its own entry.
+	statuses := make([]any, requests)
+	start := make(chan struct{})
+
+	var waitGroup sync.WaitGroup
+
+	for idx := range requests {
+		waitGroup.Add(1)
+
+		go func() {
+			defer waitGroup.Done()
+
+			<-start
+
+			resp, reqErr := doRequestAuth(
+				tserver.t, http.MethodPost,
+				tserver.url(apiMessages), token,
+				bytes.NewReader(body),
+			)
+			if reqErr != nil {
+				tserver.t.Error(reqErr)
+
+				return
+			}
+
+			defer func() { _ = resp.Body.Close() }()
+
+			var result map[string]any
+
+			_ = json.NewDecoder(resp.Body).Decode(&result)
+
+			statuses[idx] = result[statusKey]
+		}()
+	}
+
+	close(start)
+	waitGroup.Wait()
+
+	return statuses
+}
+
+// TestChannelHashcashRejectConcurrentReplay sends one stamp
+// in several requests at once and checks that exactly one is
+// accepted and the other member receives exactly one copy.
+func TestChannelHashcashRejectConcurrentReplay(t *testing.T) {
+	const (
+		channel  = "#hcrace"
+		text     = "one stamp"
+		sender   = "hcrace_sender"
+		requests = 16
+	)
+
+	tserver := newTestServer(t)
+	senderToken := tserver.createSession(sender)
+	receiverToken := tserver.createSession("hcrace_receiver")
+
+	tserver.sendCommand(senderToken, map[string]any{
+		commandKey: joinCmd, toKey: channel,
+	})
+	tserver.sendCommand(receiverToken, map[string]any{
+		commandKey: joinCmd, toKey: channel,
+	})
+
+	tserver.sendCommand(senderToken, map[string]any{
+		commandKey: modeCmd,
+		toKey:      channel,
+		bodyKey:    []string{"+H", "2"},
+	})
+
+	_, lastID := tserver.pollMessages(receiverToken, 0)
+
+	msgBody, marshalErr := json.Marshal([]string{text})
+	if marshalErr != nil {
+		t.Fatal(marshalErr)
+	}
+
+	stamp := mintTestChannelHashcash(t, 2, channel, msgBody)
+
+	statuses := tserver.sendConcurrently(senderToken, map[string]any{
+		commandKey: privmsgCmd,
+		toKey:      channel,
+		bodyKey:    []string{text},
+		metaKey:    map[string]any{hashcashKey: stamp},
+	}, requests)
+
+	accepted := 0
+
+	for _, status := range statuses {
+		if status == "sent" {
+			accepted++
+		}
+	}
+
+	if accepted != 1 {
+		t.Fatalf(
+			"expected 1 accepted message, got %d: %v",
+			accepted, statuses,
+		)
+	}
+
+	msgs, _ := tserver.pollMessages(receiverToken, lastID)
+
+	delivered := 0
+
+	for _, msg := range msgs {
+		if msg[commandKey] == privmsgCmd &&
+			msg["from"] == sender {
+			delivered++
+		}
+	}
+
+	if delivered != 1 {
+		t.Fatalf(
+			"expected 1 delivered copy, got %d: %v",
+			delivered, msgs,
+		)
+	}
+}
+
 func TestChannelHashcashNoRequirementWorks(t *testing.T) {
 	tserver := newTestServer(t)
 	token := tserver.createSession("hcnone_user")
diff --git a/internal/service/service.go b/internal/service/service.go
index 4a43946..2977aa9 100644
--- a/internal/service/service.go
+++ b/internal/service/service.go
@@ -1285,8 +1285,9 @@ func (s *Service) requireOper(
 // checkChannelHashcash returns ERR_CANNOTSENDTOCHAN unless
 // the channel has no +H, or meta carries a hashcash stamp
 // for this channel and body with enough bits that has not
-// been used before. A stamp that passes is recorded as
-// spent.
+// been used before. Recording the stamp as spent is the
+// check that it is unused, so a stamp sent in several
+// concurrent requests passes in only one of them.
 func (s *Service) checkChannelHashcash(
 	ctx context.Context,
 	chID int64,
@@ -1324,12 +1325,12 @@ func (s *Service) checkChannelHashcash(
 
 	stampKey := hashcash.StampHash(stamp)
 
-	spent, err := s.db.IsHashcashSpent(ctx, stampKey)
+	recorded, err := s.db.RecordSpentHashcash(ctx, stampKey)
 	if err != nil {
-		return fmt.Errorf("check spent hashcash: %w", err)
+		return fmt.Errorf("record spent hashcash: %w", err)
 	}
 
-	if spent {
+	if !recorded {
 		return &IRCError{
 			irc.ErrCannotSendToChan,
 			[]string{channel},
@@ -1337,13 +1338,6 @@ func (s *Service) checkChannelHashcash(
 		}
 	}
 
-	err = s.db.RecordSpentHashcash(ctx, stampKey)
-	if err != nil {
-		s.log.Error(
-			"record spent hashcash", "error", err,
-		)
-	}
-
 	return nil
 }