Since this issue is uncommon, I think disabling config caching would be best, at least to see if it makes a difference. The variable that may cause the problem is email_summary_enabled, but don’t modify the file — it should normally be in sync with the value in the database, and changing it manually may break something.
I’m not worried about what is in the file right now, but rather what happens when WordPress/Wordfence has a bunch of requests running at the same time, odd things can happen when they all try to read/write a file very close to the same time on certain hosts.
It might be unrelated to this issue, but it would at least rule out whether this is the problem. I think we have seen one other person with the same problem, so it is hard to tell what is happening.
If you can check the scheduled cron jobs in a couple days ago, and see if there are duplicates for the summary email, that may lead us in another direction — it will at least tell us if duplicates are being scheduled over time, or if all 7 just get sent from what is supposed to be one job.