Bug: Order by primary key does not work anymore if using a mssql database

Reason: mssql does not like multiple order by statements using the same column.

This problem was introduced with pull request #2343.

Possible solution:

```
pk = self.get_pk()
-if add_pk and pk:
+if add_pk and pk and pk != _order_column:
      order_by_columns.append(direction(pk))
```