**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.


#include <Processors/QueryPlan/ExpressionStep.h>
#include <Processors/Transforms/ExpressionTransform.h>
#include <Processors/QueryPipeline.h>
#include <Processors/Transforms/JoiningTransform.h>
#include <Interpreters/ExpressionActions.h>
#include <IO/Operators.h>
namespace DB
{
static ITransformingStep::Traits getTraits(const ActionsDAGPtr & actions)