It LOOKS like Revit splits the duct for you, but really it modifies the old one by changing the endpoint to the location of the break-in family, and makes a new duct form the break-in family to the former endpoint. It just happens really fast and behind the scenes so you don’t see it. You can test it yourself with the sample systems project.
Go to a 3d view and set a new 3d section box around element id 424197 so you only see minimal unwanted stuff.
Load in a break in family of choice (I went with a fire damper), and place it on the duct which you set in the section box.
Select Element ID 424197 which was the original duct - notice it starts where the previous duct started, and ends at the newly placed fire damper.
Check the Element ID of the damper - mine was 871451.
Check the Element ID of the other section of duct beyond the damper - mine was 871454, and this ran from the newly placed fire damper to the former endpoint of Element ID 424197.
Yes this FEELS as if it splits the element, but really it’s moving the old one and create a new one based on the former endpoints and the point of the new break-in family.