To the moment of posting Zammad 5.4.1 was current (security release) - by now Zammad 6.0.0 is. Please update asap.
Classic issue on that regard would be that the user accidentally bookmarked the Zammad URL with the ticket information which causes Zammad to reopen the tab every single time you open the bookmarp. Might happen when using the history too.
Hard to tell. No clue what you mean by “some tabs” as in “always the same tab for one specific user” or not.