api.Element initialize might be called 2 times in api.Control initialize method.
On php side generate setting json in administrator page at this location. Your extended a class from WP_Customizer_Control is called to_json method on it. In to_json method, WP_Customizer_Control generate default control template string here. This template string contains input html tag with your specified tag. The input html tag in template string cause wp.api.Element calling first time. If you override content_template method in your class from WP_Customizer_Control, wp.api.Control will detect the template from content_template and create html elements and replace first time wp.api.Element.
This line is wp.api.Element calling from wp.api.Control. it is located in wp.api.Control.linkElements.
This line is first time call wp.api.Control.linkElements.
This line is second time call wp.api.Control.linkElements.