How to delete multiple post meta keys with equal values?

You should be able to do this with a combination of 2 queries.

BACK UP THE DATABASE FIRST!!

The first query creates a temporary table containing the data for posts with duplicates:

CREATE TABLE temp_tbl SELECT meta_id, post_id, meta_key, meta_value, count( post_id ) AS count_id
FROM wp-postmeta
GROUP BY post_id, meta_key, meta_value
HAVING count_id >1;

The second query then deletes rows from the wp-postmeta table where the post_id, meta_key, and meta_value match the temporary table:

DELETE  pm
FROM wp-postmeta pm, temp_tbl tt
WHERE tt.post_id = pm.post_id
AND tt.meta_key = pm.meta_key
AND tt.meta_value = pm.meta_value
AND pm.meta_id <> tt.meta_id

Drop temp_tbl once you are done.

If you encounter a problem because you are deleting too many rows at once, use a LIMIT clause on the first query to limit the number of entries in temp_tbl. Then, after running the second query, drop temp_tbl and rerun again until all rows are deleted.

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