SELECT
  c1.id,
  c1.group_id,
  c1.article_id,
  c1.tstamp
FROM
  contribution AS c1
  LEFT JOIN contribution AS c2 ON (
    c2.group_id = c1.group_id AND
    c2.article_id = c1.article_id AND
    c2.tstamp < c1.tstamp
  )
WHERE
  c2.id IS NULL
ORDER BY
  article_id;