Import creates duplicates

[digikam at pupat-ghestem.net]

Hello,

Since a few month imports are creating duplicates. If I re-import 
pictures I already imported, I end up with multiple copies of the pictures.

IIRC there was an option to tell Digikam to ignore duplicates on import 
but I cannot find it anymore.

How can I avoid this annoying behavior?

Thanks,

Matthieu