DynamoDBIO fail to write to the same key in short consecution

Internally, DynamoDBIO.Write uses the batchWriteItem method from the AWS SDK to sink items. However, there is a limitation in the AWS SDK that a call to batchWriteItem cannot contain duplicate keys.

Currently DynamoDBIO.Write performs no key deduplication before flushing a batch, which could cause ValidationException: Provided list of item keys contains duplicates, if consecutive updates to a single key is within the batch size (currently hardcoded to be 25). 

To fix this bug, the batch of write requests need to be deduplicated before being sent to batchRequest.addRequestItemsEntry