Welcome to Android Central! The duplicate contacts might be from other social network accounts, like Facebook or Twitter. Go to
Settings>Accounts, select one of those accounts (like Facebook), and see if there is a checkbox for Contacts. Uncheck it if you don't want those contacts to sync.
I assume you're using the stock Email app for your Comcast email account. Do you know if the email account settings are set for a POP3 or IMAP server? You can find out by opening Email, tapping the Menu button, selecting the email account, and checking the account's server settings. If it's POP3, that often deletes the email from the server once it gets pushed to your phone. It's an old protocol. IMAP is preferable--see the comparison here:
IMAP vs POP3. Here are the Comcast/Xfinity IMAP settings:
https://xcsignup.comcast.net/imap_settings.html
If you'd rather not deal with that, consider installing the
Xfinity Connect app, which is actually pretty decent. You can use it as your exclusive email app, and can also check your voicemails, and even make wi-fi calls using your Xfinity voice number (if you have one). If you do install Xfinity Connect, you can actually disable the stock Email app and Exchange Services by going to
Settings>Apps>All, selecting Email and Exchange Services, and tapping Disable. This will prevent those apps from opening, and potentially giving you dual notifications (e.g., if the Xfinity email was coming in to both the Email app and the Xfinity Connect app).