To clone docker volumes, you can transfer your files from one volume to another one. For that you have to manually create a new volume and then spin up a container to copy the contents.
Someone has already made a script for that, which you might use: https://github.com/gdiepen/docker-convenience-scripts/blob/master/docker_clone_volume.sh
If not, use the following commands (taken from the script):
# Supplement "old_volume" and "new_volume" for your real volume names
docker volume create --name new_volume
docker container run --rm -it \
-v old_volume:/from \
-v new_volume:/to \
alpine ash -c "cd /from ; cp -av . /to"