I've had different experiences.  The instructor guide states it will override existing content, which I take to mean it will replace existing content. ."If you import content into a new course, edit the content in the new course, and later import the previous content again, the imported content will override the existing content."

https://community.canvaslms.com/t5/Instructor-Guide/How-do-I-copy-a-Canvas-course-into-a-new-course-...

I've also done this accidentally and imported duplicates of every activity and resource. 

If you already have content in an existing course, it would be best to either import with the option to select specific content. 

https://community.canvaslms.com/t5/Instructor-Guide/How-do-I-select-specific-content-as-part-of-a-co...

Or, you could copy items individually to the new course.  This guide will show how to copy a page.  It works similarly for other content, including entire modules. https://community.canvaslms.com/t5/Instructor-Guide/How-do-I-copy-a-page-to-another-course/ta-p/1085