The description for the add() method in ScrollPane says:
    /**
     * Adds the specified component to this scroll pane container.
     * If the scroll pane has an existing child component, that
     * component is removed and the new one is added.
     * @param comp the component to be added
     * @param pos position of child component (must be <= 0)
     */

However, if a child exists, it doesn't appear to ever be removed.