please note the hash unique step, which is anti-union all

select email from employees where department_id in (select 90 from dual union all select 90 from dual)

Id	_Operation	Name	Rows	Cost (%CPU)
0	_SELECT STATEMENT		19	6 (0)
1	__TABLE ACCESS BY INDEX ROWID	EMPLOYEES	10	1 (0)
2	___NESTED LOOPS		19	6 (0)
3	____VIEW	VW_NSO_1	2	4 (0)
4	_____HASH UNIQUE		2	4 (50)
5	______UNION-ALL
6	_______FAST DUAL		1	2 (0)
7	_______FAST DUAL		1	2 (0)
8	____INDEX RANGE SCAN	EMP_DEPARTMENT_IX	10	0 (0)