New Document Wizard: The creation of same name siblings is not prevented

During the testing of HIPPLUG-1010, this problem was discovered. Solving it in the plugin would mean that we have to duplicate non-trivial CMS code. Rather than doing that, I created REPO-1259 to add support for avoiding SNS to the repository (FolderWorkflow). I suggest to wait for that issue to be addressed before taking further, plugin-local steps in this respect.