DECLARE @Foo TABLE (User_ID BIGINT, Time_stamp DATETIME)

INSERT @Foo
SELECT 2454558665777080000, '1/8/2010 16:19:56'
UNION ALL SELECT 2454558665777080000, '1/8/2010 16:19:57'
UNION ALL SELECT 2454558665777080000, '1/8/2010 16:19:38'
UNION ALL SELECT 2454558665777080000, '1/8/2010 16:19:38'
UNION ALL SELECT 2481340376380600000, '1/12/2010 10:53:52'
UNION ALL SELECT 2481340376380600000, '1/12/2010 10:53:53'
UNION ALL SELECT 2481340376380600000, '1/12/2010 10:54:47'
UNION ALL SELECT 2481340376380600000, '1/12/2010 10:54:47'
UNION ALL SELECT 2481340376380600000, '1/12/2010 10:54:47'
UNION ALL SELECT 2481340376380600000, '1/12/2010 10:54:47'


SELECT
	A.User_ID,
	CASE
		WHEN A.RowNum % 2 = 0 
			THEN A.Time_stamp
		ELSE
		CASE
			WHEN DATEDIFF(SECOND, A.Time_stamp, B.Time_stamp) >= 10 
				THEN B.Time_stamp
			ELSE A.Time_Stamp
		END
	END AS Time_stamp
FROM
(
	SELECT User_ID, Time_Stamp, ROW_NUMBER() OVER (PARTITION BY User_ID ORDER BY Time_Stamp) AS RowNum
	FROM @Foo
) AS A
LEFT OUTER JOIN
(
	SELECT User_ID, Time_Stamp, ROW_NUMBER() OVER (PARTITION BY User_ID ORDER BY Time_Stamp) AS RowNum
	FROM @Foo
) AS B
ON 
A.User_ID = B.User_ID
AND A.RowNum = B.RowNum - 1