Data Idempotency for Axon Event

Architectural Concepts
1

Hello, I’m currently using an Axon Framework 4.8.0 (Free version) with PostgreSQL as the Event Store. For the DB library, I’m using Spring Boot JPA. Currently I have an aggregate which looks like this

@Aggregate
public class ProductAggregate {
    @AggregateIdentifier
     private String productId;
     private String itemName;
     private int stockAmount;
}

I also have a command which looks like this

@Getter
public class OrderProductCommand {
    @TargetAggregateIdentifier
    private String productId;

    // the idempotency key
    private String orderId;
    private int orderAmount;
}

To prevent the OrderProductCommand to be processed twice by the command handler, I have the orderId field which is unique for every command. My question is how should I implement the idempotency for this system?

I previously found a possible solution by using an additional query table which is guaranteed to have a unique ID. Every produced events are projected into that table. For every command, it would be intercepted beforehand to check if the ID already existed or not. If the order ID has already existed, the command is rejected.

Although this solution is good, it would require a redundant table for storing all the events produced. Is there any other solution that would not require an additional table?

For example, it is possible to add an additional column to the table domain_event_entry and use a unique constraint on that column, so every duplicate event would automatically be rejected and raise an exception. Is it possible to do add a column in Axon Framework?

Thanks in Advance.