DELETE FROM wp_postmeta
WHERE meta_id IN
(
  SELECT meta_id
  FROM
  (
    -- This query will return a list of all the meta rows that are duplicates and
    -- that should be removed
    SELECT
      PM.meta_id
      ,PM.meta_key
      ,PM.post_id
      ,meta_duplicates.meta_id_to_keep
      ,meta_duplicates.duplicates_count
    FROM
      wp_postmeta PM
      JOIN
      (
        SELECT
          post_id
          ,meta_key
          ,min(meta_id) as meta_id_to_keep
          ,count(meta_key) as duplicates_count
        FROM 
          wp_postmeta
        WHERE
          (meta_key like '%_base_currency%')
        GROUP BY
          post_id,
          meta_key
        HAVING
          duplicates_count > 1
      ) AS meta_duplicates ON
      (meta_duplicates.post_id = PM.post_id) AND
      (meta_duplicates.meta_key = PM.meta_key)
    -- This clause filters out the meta rows that we want to keep
    WHERE
      -- If you used MIN() above, you should check for "(PM.meta_id > meta_duplicates.meta_id_to_keep)"
      -- If you used MAX() above, you should check for "(PM.meta_id < meta_duplicates.meta_id_to_keep)"
      (PM.meta_id > meta_duplicates.meta_id_to_keep)
  ) AS duplicates_to_remove
)