Today, I renamed an existing notebook in Penultimate. Unfortunately, I chose a name of an already existing notebook, earlier created in EN. As a result, the notebook disappeared from the PU overview of notebooks. This is counterintuitive behaviour: a 'rename' suddenly seems to delete (or at least hide) an existing notebook. And I did not immediately realise what just had happened, as there was not even a warning message informing me of the duplicate notebook name.
I guess this is a bug? It could hardly be considered a feature...
What would work best in my opinion: let PU show all EN notebooks marked for PU usage (eg by assigning a specific tag in EN). Preferably, show all EN content, allowing to annotate all notes NOT created in PU. But until PU supports this kind of annotating, it would also work pretty good if PU only shows PU-created notes in those 'PU-tagged' notebooks.
Alternative: either warn the user for duplication, or make it impossible to create a same-named notebook in PU.