If you prior history is correct and you are using an IMAP account with google you should have do not make a copy clicked as google make their own copy when you send an email and place it in sent.

You can not turn off Googles copy, so it is logical to turn off Thunderbird as you do not need two copies.

re :When I send an email why is it appearing in my inbox after sending?

'Account Settings' > 'Copies & Folders' under the 'When sending messages...section. It could happen if you have put your own email address in as a Cc or Bcc. OR if you are replying to an email and have set up to 'place replies in the folder of the message being replied to'.

Check to see if you have selected any of those checkboxes.

I also use gmail and IMAP. Gmail uses the folder called 'Sent Mail' which is seen and accessed when using IMAP. Gmail is supposed to auto put a copy of email in server 'Sent Mail' folder. They also say you do not need to select the 'Place a copy in' option because you should see it automatically. However, I've always used the following with no problems and never have two copies. For imap...

• Select 'Place a copy in'
• Select 'Other' and choose 'Sent Mail on gmail email address'