Is this a gmail or outlook.com account? Both services automatically copy sent mail to the Sent (outlook.com) or Sent Mail (gmail) folders, so 'Place a copy in' in Account Settings, Copies & Folders, should be unchecked.

Gmail accounts, I just realized that the doubles show up in my unified Sent folder because Thunderbird was set up to automatically save a copy to that folder, which is redundant because Gmail always saves the Sent emails in the Sent folders. So, if I turn off "When sending messages, automatically place a copy in", will this affect how my Gmail automatically saves a copy. The other question, how do I delete the hundreds of duplicates now?

I think I answered one of my own questions. I sent a test email and now only one is in my unified sent folder, which is the one stored in Gmail's sent folder. As for how to delete the duplicates, I tried deleting the copies in one of the two Sent folder for my main email account, but that also wiped out the same items from the other Sent folder. So I had to Undo that. This doesn't make sense to me and here's why: If I turn on the option to Save a copy in Sent folder, then both Sent folders for this Account get a copy, presumably the one associated with my Gmail account by default, and the other to the second Sent folder in Thunderbird. Hence the Unified Sent folder shows two copies. Now if I turn off Save a copy in Sent folder, only one Sent folder gets a copy, which I figured must be the regular Gmail Sent folder. But apparently the two individual Sent folders are linked, because deleting items from the one that I figured was only associated with Thunderbird will delete the same ones from Gmails Sent folder as well. So it looks like there's no way for me to delete the 800 duplicate emails.

Gmail automatically copies messages sent from TB to the Sent Mail folder, so you can unsubscribe the Sent folder. Plus, for gmail, the setting in Account Settings, Server Settings, 'When I delete a message' should be 'just mark it as deleted' due to gmail's storage rules.

http://kb.mozillazine.org/Gmail

I don't understand the first part of your reply sfhowes.

Right-click the account in the Folder Pane, Subscribe..., uncheck the Sent folder and check the Sent Mail folder. Also, make the change in Copies & Folders in Account Settings.

I don't see Subscribe anywhere

Subscribe is only for IMAP accounts, the most common setup, so your account must be POP.