Difference between average session duration per session and Average session duration per user?

Here is a simple difference explained by definition:

Average session duration per session = total duration of all sessions(sec)/number of sessions

Average session duration per user = Total duration of all sessions(sec)/number of unique users

For example, the total duration of all sessions is 25000s and the number of sessions is 125

Average session duration per session = 25000/125 = 200s

And if the number of unique visitors is 250, average session duration per user would be 25000/250 = 100s.

You will see both the metrics in the behavior section while using the advanced segments in google analytics.

advanced segments google analytics

advanced segments google analytics