IF OBJECT_ID('tempdb.dbo.#tbl') IS NOT NULL
        DROP TABLE #tbl

CREATE TABLE #tbl (
[a] VARCHAR(1) NOT NULL
,[b] VARCHAR(1) NOT NULL)

INSERT INTO #tbl
(
    [a],
    [b]
)
VALUES
('1','x'),
('1','x'),
('2','y'),
('2','y'),
('3','z'),
('3','z')

DELETE tab
FROM (SELECT ROW_NUMBER() OVER(ORDER BY [t].[a]) AS row_num
    ,t.*
    FROM [#tbl] t) tab
INNER JOIN (SELECT
            [tble].*,
            RANK() OVER(PARTITION BY [tble].[b] ORDER BY [tble].[ROW_ID]) AS RK
            FROM (
                SELECT
                ROW_NUMBER() OVER(ORDER BY [tbl].[a]) AS ROW_ID,
                [tbl].*
                FROM [#tbl] AS tbl
                ) AS tble
            ) RK_TBL
            ON [RK_TBL].[a] = [tab].[a]
            AND [RK_TBL].[ROW_ID] = [tab].[row_num]
            AND [RK_TBL].[RK] > '1'

SELECT * FROM [#tbl]