i found one of the solution, if you know more, please let me know -solution is very simple, as greetings is simple list, let make it empty using this line of code :

greeting.current.controls=[]

so that updated fragment  will be this :
    def hello_here(e):
        greeting.current.controls=[]
        greeting.current.controls.append(ft.Text(f'hello {first_name.current.value} {last_name.current.value}'))
        first_name.current.value =""
        last_name.current.value =""
        first_name.current.focus()