SQL Server 2019

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create table #Test (Id_pk int identity(1,1) not null, id int not null, Flag bit not null)
    create unique index unq_test_id_flag
        on #test(id)
        where flag = 1
Insert into #Test (id, Flag) 
values (12, 0), (12,0), (12, 0), (12,0),  (12, 1), (12,1), (13,0), (13, 0), (14,1), (14,1), (20,1),  (20,0), (30,1),  (40,0)
select * from #Test
Msg 2601 Level 14 State 1 Line 1
Cannot insert duplicate key row in object 'dbo.#Test' with unique index 'unq_test_id_flag'. The duplicate key value is (12).
Msg 3621 Level 0 State 0 Line 1
The statement has been terminated.

Insert into #Test (id, Flag) 
values (12, 0), (12,0), (12, 0), (12,0),  (12, 0), (12,1), (13,0), (13, 1), (14,0), (14,1), (20,1),  (20,0), (30,1),  (40,1)
14 rows affected