First, are you getting two messages when online with website? If not, it may help to know the settings you changed that you mentioned. And it may also help to see a screenshot of the application in folder pane.

When you were using Thunderbird, you may have set a setting for saving sent messages, which was unnecessary and redundant because Outlook saves sent messages automatically. So duplicates that you are seeing now probably originated in Thunderbird. If you're not going to use Thunderbird again, just delete the duplicates in Outlook. If you are going to use Thunderbird again, check your settings in account settings > copies & folders.

Or are you getting duplicate sent messages purely within Outlook? Then that is a question for Microsoft. Make sure that Thunderbird is closed, send yourself a message in Outlook, and see.

I have this issue but the OP has not clarified the duplicate emails are in his inbox or when he sends. In my case, I see a duplicate emails in the sentbox even though I have unchecked "Place a Copy in" under "Copies & Folder".

I have this issue but the OP has not clarified the duplicate emails are in his inbox or when he sends. In my case, I see a duplicate emails in the sentbox even though I have unchecked "Place a Copy in" under "Copies & Folder".

I don't know why that would happen. Two things to try:

1. Close Thunderbird. Send yourself a message in webmail. How many copies go to the sent folder in webmail?
2. Start Thunderbird in troubleshooting mode (under the "help" menu). Send yourself a message. How many copies go to the sent folder?

Are you using any mesage filters that copy sent messages?