/*
declare @tbl table (n1 int, n2 int, stat varchar(10))

Insert Into @tbl (n1, n2) Values 
(1, 2),
(9, 4),
(9, 4),
(3, 5),
(9, 4),
(1, 7)

update t1 set stat = 'дубль'
	from @tbl t1
	join (
		select distinct n1, n2 from @tbl
			group by n1, n2
			having count(n1) > 1
	) t2
	on 
		t1.n1 = t2.n1 and t1.n2 = t2.n2

select * from @tbl
*/

class item
{
	public int n1;
	public int n2;
	public string stat;
	public item (int n1, int n2)
	{
		this.n1 = n1;
		this.n2 = n2;
	}
}

void Main()
{
	var tb = new List<item>
	{
		new item(1, 2),
		new item(9, 4),
		new item(9, 4),
		new item(3, 5),
		new item(9, 4),
		new item(1, 7)
	};
	tb.Dump("Initial state");

	tb.ForEach(i => 
					i.stat = tb.GroupBy(itm => new {n1 = itm.n1, n2 = itm.n2 })
	  						   .Where(g => g.Count() > 1 )
							   .Where(d => (d.Key.n1 == i.n1) && (d.Key.n2 == i.n2)).Any()?"дубль":"нЭт");
	
	tb.Dump("After update");
}