**u should know the complete account settings of ur Internet Service Provider**
ok. make sure mail app. is quit. now the first things first. back up all your mails.
it doesnt matter if its duplicated as of now.
goto ur home folder> library>mail. backup the complete mail folder to ur desktop. copy and paste would do. after this goto home folder> libarary> preferences> find the file com.apple.mail.plist drag it to the desktop. restart the computer and open mail.
Setup ur account and import the mails from the deskop. delete the duplicates. should be working now.