I might have a clue, I have 4 machines that all use Joplin with same Dropbox account, maybe it happens when Joplin try to import the new “Welcome!” note book folder. Deleting this notebook fixed the problem removed all the duplicated tags.
Confirmed that following @favadi's steps reproduces the same duplication behavior with a fresh install of Joplin on Mint Linux 19.1. I am using a personal WebDav on a bluehost server.
These tag duplications appear after syncing with my Joplin Android or Windows clients.
Deleting the Welcome! notebook resolves the problem locally and deletes the Welcome! notebook across my machines, but creates duplicate tabs any new notebook added on my one of my clients.