DynamoDB Stream is showing up both INSERT and UPDATE for a new record insertion

I'm seeing 2 events (INSERT and MODIFY) on my DynamoDB table which is a global table with one global secondary index.

As a result configured trigger(lambda) is executing for 2 times for single insertion, adding extra processing cost.

I couldn't see any documentation that explains about 2 events for single DB insertion.

Can anyone help me understand it?

Thanks.