With Google calendars, you can create additional calendars linked to your account. This is convenient, say, to split your own events from events of others that don’t use any online calendar but that you want to track.
With iOS 4, adding a google account would display only the primary calendar, not the auxiliarary ones. The solution then was to add them individually as WebCal calendars. The WebCal URL is somehow cryptic but could be obtained from the ID of the calendar found in the settings.
After upgrading to iOS 5, all events are duplicated n times, where n is the number of auxialliary calendars. Sounds like an aweful bug, isn’t it? Actually not, things only got better. The auxiliary calendars are now correctly supported.
Go to m.google.com/sync and select the auxiliary calendars you want to sync. The google calendars you selected will all appear under you google account on iOS 5 (maybe you need to recreate it, though). You can remove the spurious individual WebCal calendars safely.