FileIO can produce duplicates in output files

FileIO can produce duplicates in output files - depending on a runner.

Concrete example for Spark when executing as batch:

When using FileIO with specific number of shards, it will use default sharding function which is a round robin shard assignment with random seed. In multistage pipeline, data between stages are hold by shuffle service until downstream stage request it for further computations. If shuffle results computed with this seeded shard function are lost - e.g. shuffle service fails because of HW error - then Spark will attempt to recover data by computing them again from source data. As a result of a random seed sharding, this will assign different shard - and therefore key to the element.

More details are discussed in this thread:
https://lists.apache.org/thread.html/r5e91d1996479defbf5e896dca3cf237ee2d9b59396cb3c4edf619df1%40%3Cdev.beam.apache.org%3E