I am trying out Firefox Quantum and therefor using the webextensions vesion of xmarks. I am not having and issue where my bookmarks keep getting duplicated. I have to go back and find the revision and restore. I've been an xmarks user since the old days when it was called Foxmarks so I know how to handle the upload and downloads when setting up new browsers. However, after a restore and then making sure I download and overwrite on other browsers, I'm still ending up with duplicated bookmarks later on.
Also of note: When doing a restore via the webpage, it never completes. It sits there saying "restoring bookmarks..." and never goes away. However, the bookmarks have been restored.