I have added Lightning to Thunderbird. Duplicate Calendars?

I have installed the Lightning Add On to Thunderbird. It looks like a great calendar, but when I create a new Calendar it is duplicated.

If I create a Calendar called "My Calendar" one entry is shown in the Calendar panel (bottom-left of screen.) When I restart Thunderbird three entries appear all called "My Calendar". How can I fix this?

Jonathan
May 2009