  :::Really. It is just a matter of choosing less-than-or-equal in place of less-than relation. That of course complicates the search algorithm, because when we find an item 'equal' to the given key, we need to check also its siblings for equality, and corresponding subtrees as well. In a degenerate case of a tree of N 'equal' nodes we need to find and return all nodes as a result of a single search. That's difficult, so we usually would rather keep a single instance of every key, and link duplicates in some additional piece of storage, say a kind of linked list.   