I answer myself: to repair (delete duplicated bookmarks) my bookmarks on my laptop:
- Start the 2 computers (but do not start Opera on the desktop)
- Export the laptop's bookmarks to the desktop computer for example with Dukto, usb key, email, etc....
- Disconnect the Internet (offline)
- Delete all current bookmarks (Desktop)
- Import the laptop's bookmarks into Opera (desktop)
- reconnect Internet & sync again (desktop)
It's also possible to simply...
- Go offline
- delete all bookmarks on the desktop computer
3 Go online and wait for synchronization