I have four folders (plus a number of single bookmarks) on my Bookmarks bar. Each folder contains a few bookmarks.
After activating sync, this has started to happen:
Each folder now contains the original bookmarks, plus the bookmarks from another folder on the bar.
The other folder still contains its original bookmarks, plus the bookmarks from a third folder. And so on.
No bookmarks have gone missing, but there are a lot of duplicates. I have deleted the duplicates several times, but they keep coming back.
Opera 29, Windows 7.