I cannot solve the issue, but I'll share a possible cause. I presume this is a POP account. If so, there is a file named popstate.dat in the profile folder that keeps track of last downloaded message to prevent what you are describing. Now, if some process is running (antivirus?) that is preventing thunderbird from saving that file, then it never updates and keeps downloading the same messages. If you check in profile folder, in the Mail folder for the account, you will see the file. To get there: - click help>troubleshootinginformation, scroll down to 'profile folder', click 'open folder', exit thunderbird. You are now in the profile. Click the Mail folder and then the account folder to see the popstate.dat file.