Hi everyone, I believe the issue may have been resolved in the snapshot branch already. As I had mentioned before in this thread, I think that opening external links while Vivaldi had no windows open was the root cause of the problem here, but that no longer seems to trigger this bug for me in the latest snapshot.
So this could very well be fixed for good once version 6.7 is released. However, unless there is some other method that causes duplications, you may still experience this.
In that case, if you have experienced duplicated tabs in workspaces, I strongly suggest you to please give the latest snapshot a try. If you still get duplicate tabs after opening Vivaldi, try to pay very close attention to the manner in which you are opening it and do share your method here please. I'd be happy to test it out.