>I would like to consider them duplicates.

It may not be most efficient way but it should provide you with starting point. Try

SELECT * 
	FROM mytable
	WHERE CAST(SUBSTRING(col1, PATINDEX('%[0-9]%', col1), 7) AS int) IN (
SELECT NumPart 
	FROM (
		SELECT *, CAST(SUBSTRING(col1, PATINDEX('%[0-9]%', col1), 7) AS int) AS NumPart
				FROM mytable ) dt1
	GROUP BY NumPart
	HAVING COUNT(*) > 1)
	ORDER BY CAST(SUBSTRING(col1, PATINDEX('%[0-9]%', col1), 7) AS int)