Mail, import .mbox Sent Items, none related emails group together as duplicates

Dear friends

I am in deep need of help.

I am moving from Outlook 2011 to Mac OS X Lion - Mail.app.

I have imported the sent items.mbox from Outlook.

At first I thought there were missing emails as the total number of messages do not tally with the Outlook numbers.

As matter of fact, THEY ARE NOT MISSING. They have been grouped together as duplicates.

Quite a number of messages are grouped together and they are actually not RELATED MESSAGES.

I need Mac Mail developers to advise.

How can i stop or remove this relationships so that the Mail program can stop identifying them as duplicates?

it is not about the VIEW itself, it is about having an option or command to separate them permanently.

I believe this is a Mac Mail bug!!

PLEASE HELP!!!!