I might guess that there are four different account entries that all reference the same mail servers, but that's not entirely clear, nor entirely certain. It might be helpful to describe how many mail accounts are configured in Mail.app, and if they're for different servers. With a little more detail, somebody here might be able to resolve this for you — are all messages getting duplicated, for instance? Or just specific mail messages?
If this is multiple accounts, you can view a list of the accounts that have been configured:
Mail.app > Mail > Preferences > Accounts
The accounts that are configured are listed in the left column. You can then click on each one, and see which POP or IMAP mail servers are being referenced in the information shown to the right.