fix cloning

fix the issue introduced in #74 

the doc on WorkspaceInterface::cloneFrom 

     * @throws ItemExistsException if a node already exists at
     *      destAbsPath and same-name siblings are not allowed or if
     *      removeExisting is false and an identifier conflict occurs.

the thing is that jackrabbit supports same name siblings but we do not, so we need to prevent creating same name siblings in this case.