When you import an update from commons, does it replace the old one with the updates or do you get double of everything?
It depends on what you are importing. Individual things such as modules, pages, quizzes, etc. are added to their designated place (modules goes to modules and so forth). However, if you import a course it will replace old content.
Here are some guides for help:
What is Canvas Commons?
How do I use Commons?
How do I view my imported and shared resources in Commons?
Retrieving data ...