**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 (checkColumnsAlreadyDistinct(columns, input_streams.front().distinct_columns))
        return;
    if (!pre_distinct)
        pipeline.resize(1);
    pipeline.addSimpleTransform([&](const Block & header, QueryPipeline::StreamType stream_type) -> ProcessorPtr
    {
        if (stream_type != QueryPipeline::StreamType::Main)
            return nullptr;
        return std::make_shared<DistinctTransform>(header, set_size_limits, limit_hint, columns);
    });
}
void DistinctStep::describeActions(FormatSettings & settings) const