var list = new List<int>() { 11, 9, 8, 7, 6, 5, 4, 10, 8, 7, 6, 5, 4, 2 };
var list2 = list.GroupBy(x => x)
.Where(x => x.Count() == 1)
.Select(x => x.First())
.ToList();
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