Duplicate emails

For awhile now I've been noticing that I get duplicate emails appearing in my inbox on Mac Mail. I don't think I have any sort of forwarding set (from one of the accounts to the other).

See the image below for an example. You can see that the email "Day Trips" appears in the inbox of both of my email accounts. If I delete the email from one of the accounts, the other one still appears in the inbox, but when I click on it, it's completely blank. Upon quitting and restarting the Mail program, the email is then completely gone.

What could be causing this? It's got to be some simple thing in email settings, but I'm stumped...