From the logs I see the email was sent once only by Discourse:
My guess is that there was some temporary failure returned by the MTA, and then Discourse retried? Discourse logs don’t show any bounced email.
How do I turn off HTML emails? The white background is blinding me on Mojave Dark Mode…
Apparently the emails are multipart and your mail client should be able to default to plain text. Do you use apple mail ?
https://meta.discourse.org/t/plain-text-e-mail-notifications/13895/12
No, Spark, but neither allows me to use pick mime on a per sender basis, AFAICT. I do not want to switch HTML off completely just because one mailing list does not allow me to use plain text emails.
Oh, well, I will become blind and sue my employer about it… 
See, there is always something good in every thing 
The setting I was referring to would default to use plain text if available and html otherwise. So not disabling it completely. But maybe it does not exist.