How to cancel toast message on "else" condition, even if it still applies

When a user is signing up to my app, I have a condition that checks if a username is taken to prevent repeated names.

if (takenUsername == "taken") {
                   Toast.makeText(this@SignUp, "This username is unavailable", Toast.LENGTH_SHORT).show()
                }

However, if the username is free the sign-up becomes successful and that username is then added as a taken username in the DB

else{
     takenNames.child(usernameSortBy).setValue("taken")
}

the database looks like this:

name1: taken
name2: taken
name3: taken
name4: taken

The problem is that the toast message seems to apply to both cases - if it's taken then the toast appears. If it's not taken then the signup becomes successful and the toast still appears as it is NOW taken.

If sign-up is successful, I do not want that toast message to show up.