POP3? Gmail?
This is how Gmail's POP3 works. It only "shows" each message to a mail app exactly once, and then not again.
Please do not use Gmail with POP3. Use IMAP on all devices in all mail apps. Aqua has a preset for Gmail using IMAP, Outlook supports IMAP too.
Now, if you insist, feel free to try the "recent" mode:
https://support.google.com/mail/answer/47948?hl=enNote that you will need to do this in all mail apps that access the account, including Aqua, Outlook, Thunderbird, K9, etc.