Why are two random C# arrays identical? [duplicate]

I have a method that creates an array of random doubles between -1.0 and 1.0

private double[] noise(int length)
{
    Random randn = new Random();
    double[] n = new double[length];

    for (int i = 0; i < length; i++)
        n[i] = 2.0 * (randn.NextDouble() - 0.5);

    return n;
}

When I create two arrays using this method the contents arr1 and arr2 are identical.

double[] arr1 = noise(100);
double[] arr2 = noise(100);