I have looked more closely, and I’m 98% sure the problem relates to the “Send me an email for every new post (unless I mute the topic or category)” setting.
If it’s a category or thread that I’m watching, or if I’ve been @mentioned, then I get two emails. If I wouldn’t otherwise be notified about the message, but I’m only getting it because of the every new post setting, then I get just one email.
For example I was only getting one email (no duplicate) for messages in a thread until someone @mentioned me, then I get duplicate emails for that message: No activity of android client openMRS
I got just one email (no duplicate) for messages in this thread (which is in a category I don’t watch): GSOC 2016 participation
I got duplicate emails for this thread (in a category I DO watch): Unable to Fetch Locations Error -- on installing Bahmni
@michael, can we report this as an upstream bug in discourse?