Why Duplicate Mailboxes in my Sidebar?

I'm using Mail in Lion. The Sidebar shows the MAILBOXES heading and under that Inbox, Sent, Trash, etc. each with all my email accounts. ON MY MAC then has all the folders I created (when I was in Tiger, they came over when I upgraded to Lion). Also is REMINDERS and RSS, which I'd like to get rid of but apparently can't.

Then at the bottom are headings (all uppercase) with each of my mail accounts with subfolders and all the same messages duplicated from the above folders. I don't know if this is a new Lion thing or it's because I switched from a POP to an IMAP gmail account. I'm assuming it's an IMAP thing because my two POP accounts aren't showing there.

How do I get rid of these? Are they necessary? I'm hoping the messages aren't actually duplicated on my harddrive, thereby taking up twice the space. Are these just direct links to the IMAP server?

Any light shed would be appreciated.