If someone sending a mail to account A. Why am I receiving the same email in account B. I checked in 'setting->forward' but there is no relation between two accounts.it happening for each and every mail.
One way that email from multiple accounts can end up in one account besides forwarding is via settings/accounts and import/check mail from other accounts.
That section allows you to specify accounts, and what to do with the email from that account. You can add a label, archive the email (skipping inbox), leave the email on the server. That means that every email from that other account will end up visible in Gmail's "all mail" label, or even the inbox.
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.