**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.


{
    auto expression = std::make_shared<ExpressionActions>(actions_dag);
    pipeline.addSimpleTransform([&](const Block & header)
    {
        return std::make_shared<Transform>(header, expression);
    });
    if (!blocksHaveEqualStructure(pipeline.getHeader(), output_stream->header))
    {
        auto convert_actions_dag = ActionsDAG::makeConvertingActions(
                pipeline.getHeader().getColumnsWithTypeAndName(),
                output_stream->header.getColumnsWithTypeAndName(),
                ActionsDAG::MatchColumnsMode::Name);
        auto convert_actions = std::make_shared<ExpressionActions>(convert_actions_dag);
        pipeline.addSimpleTransform([&](const Block & header)
        {
            return std::make_shared<ExpressionTransform>(header, convert_actions);
        });
    }
}
void ExpressionStep::describeActions(FormatSettings & settings) const