If you put an invalid property into lFields (or remove that property from content type but don't update the edit.cfm). e.g
<ft:object typename="farBlog" ObjectID="#stobj.objectID#" format="edit" lFields="dudproperty" Legend="Categorisation" />
You end up with the first ft:object form fields repeated again in the Legend you specify. Which means when user saves it doubles up.
Suggest filtering lfields argument/attribute for non-existent fields. Then the tag machinery should work as intended.
Had tested on core 7.2.2. so now is oke