Will copying/importing course components overwrite existing course material?

The only components you might overwrite are course files. Course files are overwritten if one of the files being copied has the same name as an existing file.

No other components (Quizzes, Dropbox folders, etc.) are overwritten even if they have the same name as an existing component, however they will be duplicated. If you are unsure if you are copying over items that are already in the course site, we recommend checking the destination course site first.