>>When you said that you confirmed without question that there were no duplicates by using the web interface, I automatically assumed you were connecting to an online server
Protonmail’s web interface, mail.protonmail.com.
>>may be of some help if you would like to spend a little more time
Cool, thanks for the link…maybe I’ll come back to this at some point, but for the moment it looks like other clients will likely be the more reliable solution.