how can I find duplicate values on a list,
Let's say I got a List like this:

```
List<Map<String, dynamic>> users = [
    { "name": 'John', 'age': 18 },
    { "name": 'Jane', 'age': 21 },
    { "name": 'Mary', 'age': 23 },
    { "name": 'Mary', 'age': 27 },
  ];

```

How I can iterate the list to know if there are users with the same name?