Sometimes, an incoming message shows up twice in my Inbox. What's up?

I believe this is a server issue somewhere along the path from sender to your email server. If you look at the source of a message (press <Ctrl+U> you will see all the header information that's normally not interesting to you the recipient. That includes a number of Received From: lines representing the stops the message has made along the way. Each stop stamps the date and time, and the most recent is found at the top. There will also be a Date: line which marks the date and time the the sender's server first received the message. This is the date that Thunderbird displays. I expect that if you examine the first (top) time stamp on the list, you find that each message was received by your email server at a different time, indicating some delay upstream.

Normally a message will take less than a couple seconds to pass through an intermediary server, but network and server conditions can affect that. If you go down the list and find an unusually long gap between times, you may have found the sticking point and possibly where the message was duplicated. Not that you can do much about it.