I also have this issue. I thought it may be something to do with the sync as I use Vivaldi on a Linux machine at home. However, I have turned off the sync and the problem still arises, but only on the Mac. This is an extremely annoying bug as the duplication multiplies daily making the browser very slow. Can someone at Vivaldi please respond to this thread. It was stared a year ago and apparently has not yet been fixed. Thanks.