I have just uploaded a new version of dom3editor (0.8.1). I have added support for warning about duplicate id numbers in new objects (duplicate ids will be marked in yellow in the editor). So, if there are two new monsters with the same id, you will be warned. I did not put a warning for modifying the same object twice (selectmonster on the same id twice for example). If people think this is a good warning to add, it would be easy to add.
Also, there is now a button in the toolbar that will update any duplicate ids to use a new unique id number (within the ranges defined in the modding manual). This should fix the mod (at least as far as id numbers of new objects are concernde). Also, any references to the id will be updated. Here is a screenshot:
I have also uploaded a MacOS version with some caveats. You need to have Java (at least 1.6) installed. Plus, I don't have a Mac to test on so I don't know if it even runs. If anyone downloads it and gives it a try plase let me know if it runs or not.