[Bug]: DatastoreIO.Write fails multiple mutations of same entity on `finishBundle`

[bitnahian]

What happened?

Similar to #18521

Fails on the edge case when finishBundle is executed at the end of a bundle but flushBatch does not take into account duplicates before trying to write to Datastore.

Possible solutions include removing duplicate mutations from mutations ArrayList before flushBatch or just using a HashMap of mutations instead of maintaining an ArrayList which can ensure uniqueness.

org.apache.beam.sdk.Pipeline$PipelineExecutionException: com.google.datastore.v1.client.DatastoreException: A non-transactional commit may not contain multiple mutations affecting the same entity., code=INVALID_ARGUMENT
java.lang.RuntimeException: org.apache.beam.sdk.Pipeline$PipelineExecutionException: com.google.datastore.v1.client.DatastoreException: A non-transactional commit may not contain multiple mutations affecting the same entity., code=INVALID_ARGUMENT

Issue Priority

Priority: 2 (default / most bugs should be filed as P2)

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[bitnahian]

.add-labels gcp,io,java

[SaumilPatel03]

.take-issue