Certain attachments have many duplicate metadata records. Something must be creating these, possibly on every save. It could be that the metadata is updated with 

add_post_meta(...)

instead of with

update_post_meta(...)

both functions have the result that the new value is used and create a record if there is none yet. However add_post_meta will always create a new record, after which the old record lingers in the database and is never used again.

Tasks:

• Find out which code is creating instead of updating these records.

• Investigate if we should use a plugin or post-deploy script to clean up old records, as they might affect performance.
• Create follow up task if needed