Personal Video Database

Hello

thank you for the great PVD, i think it's the best movie organizer out there

But now i find a behaviour that i do not understand:

In my database is an invisible movie (without movie ID), that is not visible in the UI.
I assume it's been downloaded in the past (when i downloaded awards or filmography etc.)
In preferences i checked "Show selection dialog if similar records are already in the db", because i do want a clean db without duplicates (I know you can remove duplicates with db cleanup, but then i do not know which duplicates are removed; so i want to hold it clean manually. OK but that's not the problem).

Lets say, the movie is "Olympia" from L.Riefenstahl.
Before editing i save a backup "Backup X".

1. I add a new movie "Olympia" and save. No selection dialog.
2. I edit this movie and add the original title and want to save. Selection dialog pops up (Similar movie already in db). I can save the movie and there are no duplicates in the db. Great.
3. Now i restore the old backup "Backup X" and add a new movie "Olympia", add the original title and save. No selection dialog pops up.

OK, you would say: "Add a title", save, and then edit the title to get the selection dialog."
But: i found out that the selection dialog sometimes appears, but sometimes it doesn't appear.
I restore backups and by trial and error i can sometimes achieve that the selection dialog appears, but i never can be sure that it pops up.

My question:
What's the logic behind the selection dialog and what do i have to do, so that the dialog always pops up?

Thanks