Comment 35 for bug 711678

Good news, I found the bug!

The problem is that if you click Save+Close, Thunderbird will do this in exactly this sequence. The save triggers an update XMLHttpRequest which also gets sent. The subsequent close, however, will cancel the XMLHttpRequest (even though it already was sent and was received by the server). That makes the request return a bogus error, and the request gets retried. Voila, 2 events created.

I'll look into how to fix this, but any help is appreciated.