Should return the same node when we add a duplicated node

As the title of this issue mention you need to add one line of code, otherwise all the nodes we be removed
```js
if (found) { // duplicated: value already exist on the tree
        found.meta.multiplicity = (found.meta.multiplicity || 1) + 1; // <2>
        newNode = found; // new line to add
      }
```