int[] int_array = new int[] {1,4,5,4,3,5,1 };
            int[] repeat_array = int_array.GroupBy(a=>a).Where(a=>a.Count()>1).Select(a=>a.Key).ToArray();
            /*
            [0]    1    int
            [1]    4    int
            [2]    5    int

             */

------解决方案--------------------

C# code

            int[] array = new int[] { 1, 4, 5, 4, 3, 5, 1 };
            int[] newArray = (from i in array
                    group i by i into grouped
                    where grouped.Count() > 1
                    select grouped.Key).ToArray();