I know that Brendan's cloudscript solution, but please correct me if I am wrong.
After I noticed there will be a huge extra bills on execute cloudscript , I remove it. Because I am just an Indie-Game developer , but over 700,000 players in my country only for now (without backend service)
I can't imagine how much extra bills will it cause by only one fuction "prevent multiple login".... (and maybe how lag will it be to check every minutes?)
I know Playfab is design for multiple login , but I don't think any game's backend is design to allow multiple login.
If anyone is confused by this and and can't find solustion(almost 6 years) , Please go vote here : https://community.playfab.com/idea/46902/prevent-concurrent-logins.html