I have an iPhone 4 (not a 4S) with the latest iOS 7 (doesn’t support iOS 8).
When I export the month’s events using the “+ EXPORT MONTH’S EVENTS” button it works, but it makes duplicates of every calendar event. At the moment, I’m unable to test on other versions of the iPhone, but this problem is consistent and is an issue for some users (as well as myself).
I’ve confirmed that this is happening on our current stable release, so I will report this as a bug in our tickets tracker and it should be solved in one of the next few versions, I think on the version 4.0.
Sorry if I can’t solve this right now for you, since this is not a simple problem.