Github user gatorsmile commented on the issue:
https://github.com/apache/spark/pull/13483
@viirya Based on my understanding, we want to avoid having duplicate output
columns generated by our APIs. If users want to explicitly have two duplicate
output columns, they can specify them in `agg`. That For example,
```Scala
df.groupBy("col1").agg($"col1", $"col1", count("*"))
```
In addition, `spark.sql.retainGroupColumns` is an internal parameter. The
default value is `true`. That means, users might not realize they can disable
it. Thus, it makes more sense to avoid generating duplicate columns that are
caused by retaining group-by expressions.
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