Is there a way to combine bookmarks from other browsers and save only one of duplicate bookmarks?
Try a free tool like AM Deadlink to remove find duplicates and broken links in Opera 12.17, before importing them into Vivaldi.
There's probably better solutions out there, but that's one that I have and use occasionally.
The original Opera Link tool for synchronization had some problems some years ago, this created a lot of duplicate bookmarks for me and others.
When Opera closed down Link last year, and activated Sync, they added a feature for removing duplicate bookmarks during syncing. This cleaned up my bookmarks by removing a couple of hundred duplicated bookmarks. (Only within the same folder, there may still exist duplicated bookmarks within different folders).
Now I am running both Opera 12.17 and Vivaldi, and hope I can uninstall Opera later this year.