EDIT

This is a feature, not a bug in IMO. At least in additivity we allow these structures explicitly for feature trees representing biological structures and having generic terms like "leaf" which can be used at different places in the tree and which get their full meaning by following the tree path to the root.

However, using a feature tree for "biological structures" is maybe only a workaround until structures have there own type and tree structure. So I don't know if for features there are similar use cases but I guess there are.

Maybe we could add a flag to the tree if it generally allows duplicates. If the flag is false an exception should be thrown.

There is a flag now for term trees which defines if duplicates are allowed: "allowDuplicates"

But the according functionality still needs to be implemented

This is a duplicate for #8477, #8776