This may do what you need.

Create Table Testing(A nvarchar(10) not null,B nvarchar(10) not null,

C int identity)

go

CREATE UNIQUE CLUSTERED INDEX [Testing_INDX] ON [dbo].[Testing]([A],, [C]) ON [PRIMARY]

GO

insert Testing (A,B)values('A','A')

insert Testing (A,B)values('A','A ')

insert...