For a while was getting sometimes duplicates of EMAILS already received and which I had deleted. Using the Support info, I set the option to - Leave messages on the Server That helped in that I no longer received OLD messages that I had previously deleted. When I mean duplicates, it's not just the message data, but the size, date & time that are the same ! I am at the latest release, Thunderbird 31.3 Am using POP3 and my service provider is Bell Having been using Bell / Thunderbird /POP3 for many years and this is the FIRST time I have this type of problem.