First, let me say this is an amazing module! I've spent the last couple of days setting up a clean SS environment, seeing what's improved with the intent of rebuilding a site. So far, I think I've installed everything UncleCheese has written.
Anywho, I did have some stress with Event Calendar. This is the current version (r76) with no customization done yet, so out-of-the-box. It lists every event twice. I later installed the subsite module, and it lists events and announcements from a calendar on the subsite, but only once.
It looks to me like a problem in Calendar::Event(). The where clause for pulling announcements is mal-formed. The following change worked for me:
(lines 345-352, but I think this added a couple of lines. It's all the second parameter to DataObject::get())
(StartDate <= '$start' AND EndDate >= '$end') OR
(StartDate BETWEEN '$start' AND '$end') OR
(EndDate BETWEEN '$start' AND '$end')
Enjoy, and thanks for the fine module!