Switching from POP to IMAP using Outlook 2010 and Gmail

I'm going to begin using IMAP versus POP for my Gmail.  Many thousands of messages are in my Outlook 2010 and online with Gmail.  I thought these emails had been deleted using the settings "delete messages on server after 60 days" within but for some reason they weren't deleted.  I would like to keep the folder structure I've developed within Outlook.

I'll be deleting the account on my Outlook (keeping the .pst and current emails in tact) then adding the account using IMAP.  I plan to then drag the folders over to the IMAP account.  My question is, there will be lots of duplicate messages.  Is there a way to delete duplicates or is Gmail smart enough to know this message already exists and just not copy it?  Thanks in advance.
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1namylnAuthor Commented:
Ended up using this extension in Thunderbird.  Did the trick.

1namylnAuthor Commented:
Since Gmail doesn't use folder but labels instead I wonder if there is a way I can import all of the folders I have as labels?
1namylnAuthor Commented:
I'm moving the messages to the Gmail IMAP via Outlook.  My understanding was Gmail will not duplicate a message but it apparently is.
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David Johnson, CD, MVPOwnerCommented:
1. GMail doesn't delete messages following the delete from server after x days.
2. going from POP (client based) to IMAP (server based)  in your IMAP settings, you will get all of the messages on the server

1. Status: POP is enabled for all mail that has arrived since 08/06/2013
      Enable POP for all mail (even mail that's already been downloaded)
     Enable POP for mail that arrives from now on IMAP used to have this option (I think) (would be what you want)
      Disable POP
1namylnAuthor Commented:
Didn't realize that I selected my own answer for points.  I meant to assign points to David Johnson, MVP.
1namylnAuthor Commented:
Points are for your efforts.  Thank you.
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