If a title is supposed to be the de jure liege of another title, you need to put the vassal title inside the other title unless you intend to handle that in the history files. It should also be noted that the AI might get less sane about its wars if something doesn't have a de jure liege, since it seems to somewhat focus on consolidating that.
Unless you override the relevant character history file (or the whole character history directory, which needs to be done explicitly with replace_path in the .mod file), you cannot reuse an id, and you cannot have two titles with the same tag either (unless you just have it as a scope and put some extra de jure vassals in it). You can, however, create a second file in the landed_titles directory (and most other places) that can contain entirely new titles (e.g. a titular e_western_roman_empire).