Xmarks Sync duplicated bookmarks - Firefox 56.0.1, Xmarks 188.8.131.52a, Windows 7 x64
I had removed Xmarks while it was not syncing with Firefox 56, then found the beta sync version and added it.
On first sync, all my bookmarks (thousands!) were duplicated on the local pc. The bookmarks seemed to be ok on Xmarks' server as far as I could tell.
I'm pleased to see work being done on the sync issue as I tried Firefox's own sync and found it inferior to Xmarks.
Hope you get everything sorted!