How can i prevent from a firebase value updating because of same name tag , insteaad how can i create a tag ? (THIS IS FOR CREATING AN LOGIN AND SIGNUP SCREEN)

This is what you can do with the MIT FirebaseDB control Blocks.

What you do depends on whether you structure your tags as an individual item or as a list of items,

Perhaps this thread will give you some ideas Login com firebase (Salvo em listas)

or any of these similar threads https://community.appinventor.mit.edu/search?q=firebase%20login