Receive numerous calendar acceptance messages

Registration date
Friday February 27, 2015
Last seen
March 2, 2015
I have a user who sends out calender invites. This is no bid deal. Only one user who responds when he does not only does the sender get back a message that he accepted it but he gets back like all the acceptance messages from past calendar invites. I am not sure what to do. I have created a new outlook profile. But it still occurs. I'm not even sure where to look, any ideas?

Also this is my first post!
Update, iphone is on 8.1.3 and we are using office 365 with outlook 2010 and 2013. This is the only user who is having this problem.