To explain that further – let’s say you have configured both your personal laptop and work computer to download emails from your Gmail account via POP access. Now if the emails are downloaded to the laptop first, they wont be available on your work computer (and vice-versa).
While this is a time-saving feature, your emails are split across different computers. To trick your POP client into downloading Gmail emails that are already fetched by other clients, just use the recent mode in Gmail.
Open your Outlook (or other POP client) and add the word “recent:” to your gmail username. That means if your gmail address is “email@example.com” replace that with “recent:firstname.lastname@example.org”
NOTE: This trick will also work on custom domains if you are using Google Apps with Gmail.
Solution B:Use IMAP. POP3 isn't the only mail protocol in the world. If your mail service supports the Internet Message Access Protocol (IMAP), you can set up your client to use that. Rather than simply downloading, IMAP syncs your local client inbox with the one on your email service's server. That way, every computer or phone you check your mail with, provided you set up IMAP on all of them, sees the same messages.
NOTE: This would requires you to recreate the account in IMAP
Preventing duplicate Gmail IMAP messages in Mail/InboxClick here: http://echeng.com/journal/2010/09/23/preventing-duplicate-gmail-imap-messages-in-mail-app/
Source: [http://www.labnol.org] [http://www.pcworld.com] [Google.com]