Please, please, please, don't use Gmail in POP3 mode unless you really know what you're doing.
The app has automatic setup for @gmail.com in IMAP mode. Please use that.
Google's POP3 server will "show" each message to one app, once, and that's it.
This optimization has nothing to do with my app. It's a Gmail server feature to keep POP3 working even for very large mailboxes.
If you think you really need to use Gmail over POP3, please refer to Google's help pages:
https://support.google.com/mail/answer/75947?hl=en