You don't see an option to duplicate a complete folder because there isn't one (to my knowledge).
The way that I copy folders of files in Drive is as follows:
- Create a new folder wherever you'd like the duplicate folder to be.
- Navigate inside of the folder that you'd like to duplicate.
- Press Ctrl + a to select all of the contents inside of the folder.
- Right click the selection and select "Make a copy".
- Then drag the copied files (you can sort by file name if it helps since they will all start with the title "Copy of...") to your folder created in step 1.
- (optionally) Rename the copied files to remove the "Copy of..." from the start.
It is a hassle but doesn't take all that long in my experience.
Alternatively if you use Backup and Sync, you could copy the folders on your computer (with Ctrl + C) and paste them where you want (with Ctrl + V). However, this will not copy Google Doc files because they are represented as URLs.