Your Comcast email account is a POP account. Yahoo, Gmail, and most other modern services use either IMAP or Exchange servers that are far more flexible and powerful.
The nature of a POP account is to store your email until you download. At that point it will either delete the email or leave it; depending on the settings of the device you are using. If you've been leaving your email (the iPad's default settings) and something happens to the iPad to lose track of what emails are new/old, it will download them all again.
POP is a pain if you access your email from more than one device, because it does not sync back up to the server. You can never be sure to get the same emails on all devices, and when you do, you end up having to delete them individually off each device.
IMAP and Exchange make all change on the server, and sync those changes to each device that accesses the account. They are much better choices for primary email accounts in the modern multi-device world.
I strongly recommend using this type of account as your primary one and migrating off your Comcast account. If for no other reason than that if you ever change internet providers you are going to lose that email account. At that point any accounts or services using that account to contact you wont' be able to do so. This would include accounts that use that email to verify things like password changes or other important notifications.
The only suggestion I have as to deleting the old emails is to go to the webmail site for your Comcast account and delete them there. If you then delete and re-add the Comcast account on your iPad and iPhone the old emails should be gone. Otherwise you'll have to delete them one by one.
You should also dig into the settings for your Comcast account and turn on delete after download on at least one device or computer; then make sure you use that device/computer to access your email ever now and then. This should be the last device/computer you use each day, to give all the others a chance to download the emails before they are gone from the server.
And yes, they should be deleted from the server. Most POP servers have limited storage space. Eventually the account will fill up and stop working until you delete emails. Though that could be a long eventually, depending on the space available and how much email you get. Also, deleting the email off the server periodically will keep the old email thing from happening on your iOS devices again, or at least keep it from being quite so bad.
Good luck.
