SQL Server 2019

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DECLARE @Holder TABLE
    (
        HPKEY      int,
        HSOMEVALUE varchar(50)
    );
    
    INSERT INTO @HOLDER (HPKEY,HSOMEVALUE) VALUES
      (1,'abc'), (2,'abc'), (5,'abc'), (7,'xyz'), (9,'xyz');

/* VINTAGE */

SELECT SomeValue = HSOMEVALUE, MINKEY, 
      OTHER_KEYS_EXCEPT_MIN = STUFF(
        (SELECT ',' + CONVERT(varchar(11), h2.HPKEY)
           FROM @Holder AS h2 
           WHERE h2.HSOMEVALUE = h.HSOMEVALUE
           AND h2.HPKEY > h.MINKEY
           FOR XML PATH(''), 
           TYPE).value(N'./text()[1]', N'varchar(max)'), 1, 1, N'')
      FROM 
      (
        SELECT HSOMEVALUE, MINKEY = MIN(HPKEY)
        FROM @Holder GROUP BY HSOMEVALUE
      ) AS h
      ORDER BY SomeValue;
      
/* SQL SERVER 2017+ */

SELECT SomeValue = h.HSOMEVALUE, h.MINKEY, 
        OTHER_KEYS_EXCEPT_MIN = STRING_AGG(h2.HPKEY, ',')
      FROM @holder AS h2
      INNER JOIN 
      (
        SELECT HSOMEVALUE, MINKEY = MIN(HPKEY)
        FROM @Holder GROUP BY HSOMEVALUE
      ) AS h 
      ON h.HSOMEVALUE = h2.HSOMEVALUE
      WHERE h2.HPKEY > h.MINKEY
      GROUP BY h.HSOMEVALUE, h.MINKEY
      ORDER BY SomeValue;

SomeValue	MINKEY	OTHER_KEYS_EXCEPT_MIN
abc	1	2,5
xyz	7	9

SomeValue	MINKEY	OTHER_KEYS_EXCEPT_MIN
abc	1	2,5
xyz	7	9