Why does using the Random() class to create 100 random arrays create repeating arrays? [duplicate]

I am making a program that creates 100 arrays and fills them with random numbers between 1 and 100. However, it produces repeating arrays. The amount of repeating arrays is inconsistent and changes every time I run it. How can I create 100 non-repeating random arrays? Additionally, I am using this in a program where I time different sorting techniques. Why does it take different amounts of time to sort identical arrays (the ones that are supposed to be random arrays) with the same sorting method?

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int[] array = null;
        for (int i = 0; i < 101; i++)
        {
           Random r = new Random();
           int length = 15;
           array = new int[length];
           for (int n = 0; n < length; n++)
           {
              int newNum = r.Next(100);
              array[n] = newNum;
           }
           Console.WriteLine();
           Console.WriteLine("#" + (i + 1));
           PrintArray(array);

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