To prevent duplicates you must configure the alias account not to receive e-mails since they are already being received by the original account;
Tools-> Options-> tab Mail Setup-> button Send/Receive-> button Edit…-> select the alias account-> uncheck Receive mail items
You are able to login successfully webmail but not outlook. Outlook just asking PWD again and again, It means this is problem of pop3 service, just go through as following to resolve this issue.
1) vi /etc/xinetd.d/popa3d
then make change disable = yes in this file.
2) you have make same changes into /etc/xinetd.d/popa3ds file as well.
3) /etc/init.d/xinetd restart OR /scripts/courierup
then cpanel restart
then /etc/init.d/courier-authlib restart