Simple(?) mail problem

I'm using the provided mail client on my Macbook Pro with OS X 10.7.
I checked the mail preferences but don't see a switch that would cure the problem.

Problem: When viewing messages in the Inbox, it will display a message from a user and also any other messages that have the same message subject and sender. For example...if John.Smith@mycompany.com sends me an email about our "Widget Development"...the mail client will include any other mail with the same subject from that sender...even if its from a year ago.

Don't want it to do that!

Any help on how to turn this behavior off?

Jim