Github user gatorsmile commented on the issue:
https://github.com/apache/spark/pull/19653
> * in SparkSQL, the standard SQL UNION clause is rewritten as UNION +
DISTINCT
```Scala
case SqlBaseParser.UNION if all =>
Union(left, right)
case SqlBaseParser.UNION =>
Distinct(Union(left, right))
```
We follow ANSI SQL if you rewrite it to SQL
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