diff --git a/api/pubsub_download.go b/api/pubsub_download.go
index 687309a..e47d09f 100644
--- a/api/pubsub_download.go
+++ b/api/pubsub_download.go
@@ -96,26 +96,28 @@ func (api *API) PubsubStartDownloadSubreddit(ctx context.Context, params PubsubS
 		return errs.Wrapw(err, "failed to verify subreddit existence", "params", params)
 	}
 
-	err = api.withTransaction(ctx, func(exec bob.Executor) error {
-		_, err := api.scheduleSet(ctx, exec, ScheduleSetParams{
-			Subreddit: subreddit.Name,
-			Status:    ScheduleStatusEnqueued,
+	api.lockf(func() {
+		err = api.withTransaction(ctx, func(exec bob.Executor) error {
+			_, err := api.scheduleSet(ctx, exec, ScheduleSetParams{
+				Subreddit: subreddit.Name,
+				Status:    ScheduleStatusEnqueued,
+			})
+			if err != nil {
+				return err
+			}
+
+			payload, err := json.Marshal(subreddit)
+			if err != nil {
+				return errs.Wrapw(err, "failed to marshal subreddit")
+			}
+
+			err = api.publisher.Publish(downloadTopic, message.NewMessage(watermill.NewUUID(), payload))
+			if err != nil {
+				return errs.Wrapw(err, "failed to enqueue reddit download", "params", params)
+			}
+			return nil
 		})
-		if err != nil {
-			return err
-		}
-
-		payload, err := json.Marshal(subreddit)
-		if err != nil {
-			return errs.Wrapw(err, "failed to marshal subreddit")
-		}
-
-		err = api.publisher.Publish(downloadTopic, message.NewMessage(watermill.NewUUID(), payload))
-		if err != nil {
-			return errs.Wrapw(err, "failed to enqueue reddit download", "params", params)
-		}
-		return nil
 	})
 
-	return nil
+	return err
 }