C# Checking if there is same random value in the array or not

I want to check if there is same value in the array or not as I mentioned in the title. And if there is, I want to pass that value and check another random value to add to listbox.

In my form, there is 2 textBox, 1 listbox and 1 button. When button is clicked, listbox has to show random numbers up to sum of textbox1 and textbox2. For instance;

5 entered from textbox1 and 10 entered from textbox2. Sum is of course 15 and listbox has to show 15 random numbers but those numbers have to be different from each other.

I wrote something like that and used Contains method to check if there is same value or not. But the program froze and didn't give any error.

int a, b;
    Random rnd = new Random();
    int[] array;
    private void button1_Click(object sender, EventArgs e)
    {
        a = Convert.ToInt32(textBox1.Text);
        b = Convert.ToInt32(textBox2.Text);
        int c = a + b;
        array = new int[c];

        for (int i = 0; i < array.Length; i++)
        {

            int number = rnd.Next(c);

            foreach(int numbers in array)
            {
                if (array.Contains(numbers))
                {
                    i--;
                }

                else
                {

                    array[i] = number;
                    listBox1.Items.Add(array[i]);
                }
            }              

        }

I also did it without foreach(Only Contains part I mean). Also didn't work. I wrote in "else";

 array[i] += number; 

it also didn't work.

I would be very appreciated if you help me. Thanks in advance.