You are now given an array of user logs that contain a user and a duration. Calculate the average duration per user.

[
{"user": "jim", "duration": 10},
{"user": "bob", "duration": 15},
{"user": "carol", "duration": 56},
{"user": "jim", "duration": 49},
{"user": "bob", "duration": 145},
{"user": "stacy", "duration": 65},
{"user": "carol", "duration": 72},
{"user": "joe", "duration": 3},
{"user": "jim", "duration": 100},
]