Distinct Value from two array

string[] a1 = { "A", "B", "A", "C", "C" };
string[] a2 = { "A", "E", "C", "D", "C" };
// var unique = a1.Union(a2);
string[] a3 = new string[a1.Length + a2.Length];
a1.CopyTo(a3, 0);
a2.CopyTo(a3, a1.Length);
ArrayList arr = new ArrayList();

for (int k = 0; k < a3.Length; k++)
{
arr.Add(a3[k].ToString());
}
for (int i = 0; i < arr.Count - 1; i++)
{
for (int j = i + 1; j < arr.Count; j++)
{
if (arr[i].Equals(arr[j]))
{
arr.RemoveAt(j);
}
}
}
for (int i = 0; i < arr.Count; i++)
{
arr.Sort();
Console.WriteLine(arr[i]);
}
Console.ReadLine();