You might also find your answer in the LastPass Enterprise Manual.
Can a user be added more than once to a Shared Folder?
Yes, a user can be added to a shared folder more than once. This includes being added by their membership in a group, so be careful when considering permissions.
If a user is added more than once to a folder via multiple groups or individually multiple times with different permissions, the most restrictive settings take priority. If a user is added to the folder individually and via user groups, the individual permission would apply.
You will still see a notification from your browser once the change attempts to sync to the server and the restriction is triggered.