m = mutableMapOf<String, Int>()
m.put("a", 1)
m.put("a", 1) // throw exception here

class OneMap<K, V> {
    private val values = mutableMapOf<K, V>()
    operator fun get(key: K) = values[key]
    operator fun set(key: K, value: V) {
        require(key !in values)
        values[key] = value
    }
}

fun main() {
    val m = OneMap<String, Int>()
	m["a"] = 1
	m["a"] = 1 // throw exception here
}