#1
SELECT offset, COUNT(offset)
FROM log
GROUP BY offset
HAVING COUNT(offset) > 1;
#2
SELECT A.id, A.ts, CONVERT(TIMEDIFF (B.ts,A.ts),CHAR) AS timedifference,a.offset,a.input_offset
FROM log A INNER JOIN log B ON B.id = (A.id + 1)
ORDER BY TIMEDIFF (B.ts,A.ts) DESC