**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.


{
    if (collect_processors)
    {
        QueryPipelineProcessorsCollector collector(*pipelines.front(), this);
        transformPipeline(*pipelines.front());
        processors = collector.detachProcessors();
    }
    else
        transformPipeline(*pipelines.front());
    return std::move(pipelines.front());
}
void ITransformingStep::updateDistinctColumns(const Block & res_header, NameSet & distinct_columns)
{
    if (distinct_columns.empty())
        return;
    for (const auto & column : res_header)
    {
        if (distinct_columns.count(column.name) == 0)
        {
            distinct_columns.clear();
            break;
        }
    }
}
void ITransformingStep::describePipeline(FormatSettings & settings) const
{
    IQueryPlanStep::describePipeline(processors, settings);
}
}