@leocg I've also tried that several times, like: reset passphrase, delete opera appdata (new profile), fresh os install, etc...
Sync is enabled on my 2 PCs and my phone. But duplicated tabs are only on my PCs, no problem on my phone.
To confirm that this issue is present and can be reproduced, I've tested on 2 VMs to show you that. Even with a fresh new OS install, new opera install (so no opera appdata created) and new account (no data synced), duplicated tabs happens. It's not an issue related to previous created data/profile or updates.