Comment 3 for bug 1006136

Each Shotwell import (whether manual or via auto-import) creates a fresh set of events. So if an import includes a photo taken on May 4, and then a following import includes another photo taken on May 4, then you will see May 4 twice in the sidebar. (You can easily merge these events if you like.)

This behavior was originally by design. We've come to feel that it's more confusing than helpful, and that it would be better if imported photos were placed into existing events so that each day would only appear once. This ticket tracks this issue upstream:

http://redmine.yorba.org/issues/3565