The core requirements are to prevent duplicate processing of orders by different sales agents and to avoid adding complexity. Pub/Sub's exactly-once delivery feature is designed for this specific use case. By enabling this feature on the pull subscription, the Pub/Sub service guarantees that a message is delivered to a subscriber application only once. This prevents two different agents from receiving and processing the same order message. This solution is a native feature of Pub/Sub, requiring only a configuration change on the subscription, thereby adhering to the constraint of not adding complexity to the workflow.

A. Create a transactional database that monitors the pending messages.

This adds a new, complex component (a database) and application logic to manage message state, which violates the requirement to not add more complexity.

B. Create a new Pub/Sub push subscription to monitor the orders processed in the agent's system.

This is a reactive monitoring solution, not a preventative one. It doesn't stop duplicate processing from occurring; it only reports on it after the fact and adds complexity.

D. Use a Deduplicate PTransform in Dataflow before sending the messages to the sales agents.

Introducing a Dataflow pipeline is a significant architectural change and adds substantial complexity, which is inappropriate for a manual agent workflow and violates a key constraint.

1. Google Cloud Documentation - Pub/Sub, "Exactly-once delivery": "When you enable exactly-once delivery, Pub/Sub provides the following guarantees... A message is delivered once and only once to the subscriber clients... If a message is sent to a subscriber client and the acknowledgment for the message is lost, Pub/Sub resends the message. However, Pub/Sub does not resend the message if the acknowledgment was successfully processed by the Pub/Sub server." This directly supports using the built-in feature to prevent duplicate delivery.

2. Google Cloud Documentation - Pub/Sub, "Create subscriptions": The process for enabling exactly-once delivery is shown as a simple checkbox or flag (--enable-exactly-once-delivery) during subscription creation. This confirms it is a low-complexity configuration change, not a major architectural addition. See the section "Create a subscription with exactly-once delivery".

3. Google Cloud Architecture Center - "Exactly-once processing with Cloud Dataflow and Pub/Sub": This document describes using Dataflow for deduplication in large-scale streaming pipelines. The complexity described (windowing, state management) illustrates why this approach (Option D) is overly complex for the scenario in the question, which involves manual agent interaction.