How I am able to redefine Val in kotlin in the below program in description? Why does this program works?

Question

In kotlin the function argument is Val by default, but i re initialized the val argument successful inside function body. We can redefine only Var variable. But still this program worked. How?

The program is :
fun shippingCost(amount: Double, international: Boolean): Double {
val amount:Double = when(international){
false -> { if(amount>=75.0) 0.0 else (amount*0.1)
}
true -> { if((amount*0.15)>50.0) 50.0 else (amount*0.15)
}
}
return amount
}
fun main(args: Array<String>) {
val total = readLine()!!.toDouble()
val international = readLine()!!.toBoolean()
println(shippingCost(total, international))
}

Answer 1

In this code, the variable (val amount) is defined once, depending on the condition associated with the function argument, which has the same name (amount), about which a warning is displayed.