Syncing IPhone 5 & Outlook Calendar

I sync my IPhone 5 and my Outlook Calendar on my PC via USB. Whatever is contained in my IPhone calendar is copied over to my PC - good! However, it appears that if I have manually put in an appointment on my IPhone, and my PC has another appointment listed at the same time and date, then my PC calendar overwrites what is on my IPhone? How do I get the phone & pc to sync all the appointments from the one device to the other and vice-versa without it overwriting the other entry?