In JS, multiple prototype assignments give duplicate identifier error

```js
function A() {
    this.x = 1
}
function Am(n) { }
if (!!!true) {
    A.prototype.m = Am
}
else {
    A.prototype.m = function() { }
}
```

**Expected behavior:**
Multiple assignments should be allowed, but there should a type error on the second assignment because the types of `m` do not match.

**Actual behavior:**
Duplicate identifier error on both declarations of `m`.