“Incoming server already exists” is an occasional hiccup you get trying to configure a new email id.
Without fiddling with your inf settings, you can easily add your email account manually by adding a fake account and clicking on manually change settings.
Here is how
1. Click on Tools >> Account settings
2. Account Action ( Button at the bottom) >> add email account
3. Put here some information like
Name :email@example.com ( on anything rubbish)
Email : firstname.lastname@example.org
Password : kaflsdjklsfdjl
4. Click on Continue
5. Wait till it says thunderbird failed to find settings for your email account
6. Click on Manual Setup
7. Now you will see in your account list that the new account is created already. Fill it up with your correct server information.
That’s it. What you just did was created a fake profile and edited it later.