Tried thus query with "SELECT *" instead of "DELETE FROM" and it worked perfectly.
DELETE FROM `80dage_garmin_track` t1 WHERE EXISTS (
SELECT 1
FROM `80dage_garmin_track` t2
WHERE t1.Length = t2.Length
AND t1.Time = t2.Time
AND t1.idgarmin_track > t2.idgarmin_track
)
MySQL Error: .#1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 't1 WHERE EXISTS (SELECT 1 from 80dage_garmin_track` t2 WHERE t1.Le' at line 1