So there is a "duration" metric type with the example query

SELECT
  user_id as user_id,
  anonymous_id as anonymous_id,
  duration as value,
  received_at as timestamp
FROM
  sessions

What happens if there are several entries with the same user id? Do we sum all the values of the same user or calculate their average?

It does a SUM when there are multiple rows

Great, thanks! Just exactly what I need