I have several Google accounts on my Huawei Y6 (Android 5.1.1)

The other day one of them became unable to send mail (SMTP authentication errors). It is still receiving email.

I can still send mail from gmail on desktop, so nothing has changed with the account itself.

I figured I needed to go in and edit the server settings - only that account was missing from the list of google accounts.

I re-added it, went through authentication etc. and guess what - now I'm getting every email twice and I'm still getting smtp errors every 5 minutes from the "invisible" account.

How to fix?