Go to your calendar on icloud.com and add any calendar events that are on your iPhone that are not on your calendar on icloud.com. After your calendar on icloud.com contains all your events from your phone, go to Settings>iCloud on your phone, turn Calendars to Off, choose Delete from My iPhone when prompted, then after a minute or so turn Calendars back to On. Next, check Settings>Mail,Contacts,Calendars>Default Calendar on your phone to be sure you have selected a calendar listed under your iCloud account name as your default. Then add a new event to the calendar on your phone, and see if it appears on your calendar on icloud.com after a minute or so. Assuming it does, check your iCloud calendars in Outlook on your computer to see which calendar the new event appears on. If it appears on one of the iCloud calendars in Outlook, uncheck the other one to hide it.
Now add a new event in Outlook on the calendar your phone was syncing with and confirm that it apears on icloud.com and on your iPhone. If so, your iCloud calendar should be syncing proplerly now, and you should only see a single, unduplicated calendar in Outlook on your computer. Is that the case?