I too was receiving duplicate emails on my phone. I had previously turned on forwarding in my gmail account.
My solution: I simply went back to the gmail account where I had turned forwarding on, and left the option “forward a copy of incoming mail to *** Email address is removed for privacy ***” untouched, and selected from the pull down menu “and Archive Gmail’s copy”. Problem solved. No more duplicate emails in my inbox.
Elegant no XD.
From browser in Gmail> settings>see all settings> Click “Forwarding and POP/IMAP”> Enable POP> use drop down menu to select Archive Gmail’s copy. That stopped all mine.
Before I got that to work, I simply made a filter for each one of my 2 Gmail accounts. Then I’d have anything sent from each account to the other filtered. Simple work around. Not perfect, but at-least you can send them elsewhere and delete later on.