Most people who have more than just one mail address and forward everything to one single mailbox will run into this problem.
On your iOS device you can’t setup email accounts just for sending.
Even if you set syncing to manual you will receive the mails for this account when you open the stock mail app.
Now if you forward all addresses to one but have all accounts configured on your device (to be able to send from them) you will likely end up seeing mails doubled in the overview.
Apple doesn’t provide a way in iOS to get around this …
However, there are some solutions!
I’ll show you one way which will work in nearly every case.
There is only one dependency: You must be able to send mails over SMTP with the specific account, but it’s unlikely you aren’t able to. Check the help of your mail provider.
So I’ve three mailboxes:
Every mail I get to email@example.com and firstname.lastname@example.org is forwarded to email@example.com which is my main mailbox and setup just as normal on the iOS device.
The other two addresses I’ve setup as POP-accounts, because here I can change the server for incoming mails to something other than it should be.
I think you already get the point.
But to solve this in a fine way I needed one more mailbox where I don’t get any emails.
So I took a random mail-address at a random provider.
The data of this mailbox is what I’ve entered for the incoming server at the second and third account.
As I won’t get any mails to this account it will stay empty.
If I open the mail app now, of course it will query all accounts which are setup.
But it will only receive mails of the primary account because there are no mails on the other two.
The only minus of this solution is that I have to take care from which account I’m sending mails (new, answering, forwarding), because by default the primary one is selected.