[Solved] Why is .contains returning false when I add a duplicate? [closed]

See the documentation of List#contains given below:

Returns true if this list contains the specified element. More
formally, returns true if and only if this list contains at least one
element e such that Objects.equals(o, e).

Thus, you need to override equals method in class City. You can do it as follows:

@Override
public boolean equals(Object obj) {
    City other = (City) obj;
    return Objects.equals(name, other.name) && Objects.equals(country, other.country);
}