They are telling me it is a known issue with IMAP accounts the technicians are working on it with no ETA. But clearly, it's been going on for years. I have had this issue for at least 1 year and on 2 different devices. Most issues occur going from WIFI to Data ie leaving me house I get up the road and get a whole bunch of email notifications. But it also happens randomly throughout the day, or not for a few days. Then all day. its ridiculous
I think your post saying the issue is with IMAP accounts has fixed the issue for me after several months of trying!
I had 5 Gmail accounts an exchange account and 1 IMAP ISP account on Outlook for Android and I would get repeat email notifications seconds after reading them.
After removing the 1 ISP IMAP account I haven't seen the issue again, I will report back if it returns.