Try this :
SELECT *
FROM YourTable
WHERE LEFT(stringColumn, 5) IN (
SELECT LEFT(stringColumn, 5)
FROM YOURTABLE
GROUP BY LEFT(stringColumn, 5)
HAVING COUNT(*) > 1
)
This selects the first 5 characters, groups by them and returns only the ones that happen more than once.
Or with Substring:
SELECT * FROM YourTable
WHERE substring(stringColumn,1,5) IN (
SELECT substring(stringColumn,1,5)
FROM YOURTABLE
GROUP BY substring(stringColumn,1,5)
HAVING COUNT(*) > 1)
;