Union Vs Concat in Linq

I have a question on Union and Concat.

var a1 = (new[] { 1, 2 }).Union(new[] { 1, 2 });             // O/P : 1 2
var a2 = (new[] { 1, 2 }).Concat(new[] { 1, 2 });            // O/P : 1 2 1 2

var a3 = (new[] { "1", "2" }).Union(new[] { "1", "2" });     // O/P : "1" "2"
var a4 = (new[] { "1", "2" }).Concat(new[] { "1", "2" });    // O/P : "1" "2" "1" "2"

The above result are expected, but in the case of List<T> I am getting the same result from both Union and Concat.

class X
{
    public int ID { get; set; }
}

class X1 : X
{
    public int ID1 { get; set; }
}

class X2 : X
{
    public int ID2 { get; set; }
}

var lstX1 = new List<X1> { new X1 { ID = 10, ID1 = 10 }, new X1 { ID = 10, ID1 = 10 } };
var lstX2 = new List<X2> { new X2 { ID = 10, ID2 = 10 }, new X2 { ID = 10, ID2 = 10 } };
        
var a5 = lstX1.Cast<X>().Union(lstX2.Cast<X>());     // O/P : a5.Count() = 4
var a6 = lstX1.Cast<X>().Concat(lstX2.Cast<X>());    // O/P : a6.Count() = 4

But both are behaving the same incase of List<T>.

Any suggestions please?