Objects cannot have duplicate keys. If you assign a key to an object when that object already has said key, or write an object initializer with duplicate keys, the prior value at that key will be overwritten:

const obj = {
  foo: 'foo',
  bar: 'bar',
};

obj.foo = 'new';
console.log(obj);

const obj = {
  foo: 'foo',
  bar: 'bar',
  foo: 'new',
};

console.log(obj);

When using object spread, the same sort of thing is going on. If you add a key-value pair when a prior key already exists in the object initializer (by spread or otherwise), the prior value will be overwritten.

let happyBirthday = {...original, age: 27};

is very similar to

// Create a new object with all properties of `original`:
let happyBirthday = Object.assign({}, original);
// Assign to the `age` property of the new object:
happyBirthday.age = 27;