**Describe the bug**
ClickHouse version: 20.8.2.3
`SELECT DISTINCT b FROM (SELECT a, b FROM d GROUP BY a, b)` may return duplicate result, here is the step to reproduce:

```sql
CREATE TABLE test_local ON CLUSTER xxx (a String, b Int) Engine=TinyLog;
CREATE TABLE test ON CLUSTER xxx (a String, b Int) Engine = Distributed('xxx', 'default', 'test_local', b);
INSERT INTO test VALUES('a', 0), ('a', 1), ('b', 0)
SELECT DISTINCT b FROM (SELECT b FROM test GROUP BY a, b)
```
Expected result:
0
1
Actual result:
0
0
1

**Possible reason:**

`DistinctStep::checkColumnsAlreadyDistinct` return true while `b` exists in its front inputstream's `distinct_columns`(which is `a` and `b` in previous SQL), so `DistinctStep` is skipped in the final execution pipeline. As the comment of  `DataStream::distinct_columns` says: "Tuples with those columns are distinct. It doesn't mean that columns are distinct separately.", `DistinctStep::checkColumnsAlreadyDistinct` may return wrong result.


{
    return ITransformingStep::Traits
    {
        {
            .preserves_distinct_columns = !actions->hasArrayJoin(),
            .returns_single_stream = false,
            .preserves_number_of_streams = true,
            .preserves_sorting = !actions->hasArrayJoin(),
        },
        {
            .preserves_number_of_rows = !actions->hasArrayJoin(),
        }
    };
}
static ITransformingStep::Traits getJoinTraits()
{
    return ITransformingStep::Traits
    {
        {
            .preserves_distinct_columns = false,
            .returns_single_stream = false,
            .preserves_number_of_streams = true,
            .preserves_sorting = false,
        },
        {
            .preserves_number_of_rows = false,
        }
    };
}
ExpressionStep::ExpressionStep(const DataStream & input_stream_, ActionsDAGPtr actions_dag_)