DynamoDB TransactWriteItem Concurrency Issue

Our system uses DynamoDB Streams to trigger a handler that creates two records in Table B whenever a new record is added to Table A

Table "B" looks like that
each B#root record should have another parent# record to make sure of the uniqueness of the name of this item with the parent.
The parent is 8954 which we get from the stream of the table A

However, we have noticed that sometimes DynamoDB Streams sends two very closely timed events for a new A record creation, causing our handler to create three B records instead of two. We suspect there is a concurrency issue, but we are not sure how to solve it.

the handler will do the following code twice.

HashMap<String, AttributeValue> rootItem = new HashMap<String, AttributeValue>();
    rootItem.put("hashKey", new AttributeValue("B#"+UUID.randomUUID().toString()));
    rootItem.put("sortKey", new AttributeValue("B#root"));
    rootItem.put("name","Default");
    rootItem.put("parentValue",<Id given from dynamodb streams>+";Default" );
   

HashMap<String, AttributeValue> parentItem = new HashMap<String, AttributeValue>();
    parentItem.put("hashKey", new AttributeValue("Parent#"+<Id given from dynamodb streams>+";Default"));
    parentItem.put("sortKey", new AttributeValue("Parent#"+<Id given from dynamodb streams>+";Default"));


 Put bRootItem = new Put()
      .withTableName("B")
      .withItem(rootItem);

    String conditionalCheckExpression = "attribute_not_exists('hashKey') AND attribute_not_exists('sortKey')";

    Put bParentIrem = new Put()
      .withTableName("B")
      .withItem(parentItem)
      .withConditionExpression(conditionalCheckExpression);

 Collection<TransactWriteItem> actions = Arrays.asList(

      new TransactWriteItem().withPut(bRootItem),
      new TransactWriteItem().withPut(bParentIrem));

and that will result in the following records!

I think there is a concurrency issue here right ?

Our goal is for the handler to be idempotent and only create two B records at most for each A record. We added a condition expression to the Put operation to prevent duplicate creation of a parent item, but it does not seem to be working as expected.

What I am expecting is just 2 records

These:

or

These

Can you explain why this is happening and suggest a solution to our problem?

Its impossible that a transaction which contains a condition check on the existence of an item which evaluates to false would still create the child. Transactions are ACID compliant and the atomic nature would ensure that the entire transaction failed.

It may be possible that another process is adding children, which you are not aware of. In that case my suggestion would be to enable dataplane logging on the table and evaluate all actions pertaining that item, the logs will show you more info on where the API calls originated so you can debug further.