Ran into this issue today as well. With Lastpass change I suspect many people will have this issue importing data.
I have solved the issue however using the official bw-cli tool and a little python.
I put the gist up here for anyone that needs it if you came here trying to figure out how to delete multiple folders. https://gist.github.com/voice1/58159602be2d9ef6cdb72b8c2224ca72
You may also find the following useful for deduplication:
Bitwarden Duplicate Entries Remover : Bitwarden (reddit.com)