Tried searching to see if this was already asked. And I have stopped any kind of duplicate notification issues via the 'Act on new mails' option.

What I am writing about was possible with POP3 where all e-mail was saved locally but I have not found an easy way to do the same with IMAP. Instead I have 2 inboxes (local will never have anything in it for now). I would like to have it so even if I have a lot of IMAP, POP3, etc accounts, they all get aggregated into the local inbox directory, then filtered from there. I would happy dump Gmail's web interface altogether and only use KMail if I could get this to work (and also apply the same filter back to Gmail so my phone doesn't get a million emails).

Please see screenshot for explanation:

I don't think this is possible at the moment. You may want to file a wishlist request on bugs.kde.org.