The most common cause of multiple e-mails is leaving copies of your e-mail on the mail server.
Every time your e-mail program checks for new messages it make a connection to your mail server and has look at what is there and compares it to what you already have on your computer.
After it makes that check it is supposed to download only the new messages. However, if the mail headers stored on your computer become malformed or corrupt your mail program does not know you already have the message and downloads it again.
There is a quick check you can perform to see if this is the case. If you have Web access to your mail (most Internet providers do) then you can go to the Web and log into your e-mail. If you only see one copy of the message in Web mail but you see multiple copies in your e-mail program you can be sure that the problem is with your mail program.
The quick fix for this is to change the setting that causes your mail program to leave a copy of mail on the server.
In Outlook 2003 you do this by selecting Tools, E-mail Accounts and choosing "View or change existing e-mail accounts" and clicking Next.
Highlight the entry for your mail account and select Change then click More Settings. Under the Advanced tab there is a check box next to Leave a copy of messages on the server. If you uncheck it and then click OK, Next and Finish this will fix the problem.
The steps for Outlook Express are very similar.
The next time you check mail your messages will be removed from the server and each time you check mail you will pull down the new messages only once.
Most Internet providers prefer that you set your e-mail program to do this anyway so as to keep from filling their server to overflowing with customer e-mail messages. Roadrunner imposes a 10 megabyte limit and you will stop receiving e-mail altogether if you go over that limit.
I recognize that there are situations where having your mail stored on the server is useful. If you need to leave a copy of your e-mail on the server you can try creating a new folder in Outlook under your Inbox and move all of your Inbox messages to that folder and then check your e-mail. This might clear up the problem.