Hello, there. Run into the problem with DISTINCT expression. It works incorrectly on the main query if it was already applied to one of the subqueries bond with UNION ALL.

**How to reproduce**
Server version: 20.5.4.40. 19.17.6.36 was free of the bug.
Using Datagrip for running queries.
Non-default settings:
```
input_format_defaults_for_omitted_fields: 1
decimal_check_overflow: 0
joined_subquery_requires_alias: 0
```

Consider following query
```
select distinct number
from ((select number from numbers(10)) union all (select number from numbers(20)));
```
It works fine, returning 20 rows as expected.

But if we add distinct to subqueries (it doesn't matter to the first, the second or both)
```
select distinct number
from ((select distinct number from numbers(10)) union all (select distinct number from numbers(20)));
```

Then in 20.5.4.40 we'll see 30 rows with numbers from 0 to 9 duplicated.
However, `count(distinct number)` returns the correct number of rows.

{
    
    UInt64 unused_column = 0;
    String unused_column_name = toString(unused_column);
    while (source_columns.count(unused_column_name))
    {
        ++unused_column;
        unused_column_name = toString(unused_column);
    }
    select_query->setExpression(ASTSelectQuery::Expression::GROUP_BY, std::make_shared<ASTExpressionList>());
    select_query->groupBy()->children.emplace_back(std::make_shared<ASTLiteral>(UInt64(unused_column)));
}
void optimizeGroupBy(ASTSelectQuery * select_query, const NameSet & source_columns, const Context & context)