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...
delete all bookmarks on the desktop computer
3 Go online and wait for synchronization