Finding non duplicate element in an array

MyDict.GroupBy(x => x.Value).Where(x => x.Count() > 1)

var lookup = MyDict.ToLookup(x => x.Value, x => x.Key).Where(x => x.Count() > 1);

foreach(var item in lookup)
{
    var keys = item.Aggregate("", (s, v) => s+", "+v);
    var message = "The following keys have the value " + item.Key + ":" + keys;
    Console.WriteLine(message);
}

System.out.println("Input elements in array" +size);
for(int j = 0;j<size;j++){
    System.out.println("Enter a number: ");
    a[j] = sc.nextInt();
}

int[] a; 
int[] b;

Scanner sc = new Scanner(System.in);
System.out.println("Enter size of array");
int size = sc.nextInt();

a = new int[size];
b = new int[size];

System.out.println("a[] " + Arrays.asList(a));

for(int j = 0;j<size;j++){
    System.out.println("Enter a number: ");
    a[j] = sc.nextInt();
    b[j] = a[i];
}

int dupCount = 0;
for (int i = 0; i < size; i++){
    for (int j = 0; j < size; j++){
        if (b[i] == a[j]){
            Duplicate = a[j];
            System.out.println(Duplicate);
            dupCount = 0;
        }
    }
}
if (dupCount == 0)
    System.out.println("No common variable");