从阵列中删除重复项

我有一个加载文本文件并将其保存为数组的代码。该数组包含一个数字列表，其中一些数字是重复的。在我的代码中，我首先循环遍历数组并用-1替换所有重复的数字。然后我打算从数组中删除所有的-1值。然后将剩余的数组值(非重复项)复制到要输出的新数组中。

然而，当我试图从数组中删除-1值时，我一直得到一个错误(参见下面的代码)。我不知道为什么会发生这种情况，所以如果有人知道任何事情，请让我知道!

注:这是学校的项目，所以我只能使用循环和if语句，不能使用LINQ或foreach等。

public partial class Form1 : Form
{
    //Global Variable
    int[] Original;
    public Form1()
    {
        InitializeComponent();
    }
    //Exit Application
    private void mnuExit_Click_1(object sender, EventArgs e)
    {
        this.Close();
    }
    //Load File
    private void mnuLoad_Click_1(object sender, EventArgs e)
    {
        //Code to Load the Numbers From a File
        OpenFileDialog fd = new OpenFileDialog();
        //Open the File Dialog and Check If A File Was Selected
        if (fd.ShowDialog() == DialogResult.OK)
        {
            //Open File to Read
            StreamReader sr = new StreamReader(fd.OpenFile());
            int Records = int.Parse(sr.ReadLine());
            //Assign Array Sizes
            Original = new int[Records];
            //Go Through Text File              
            for (int i = 0; i < Records; i++)
            {
                Original[i] = int.Parse(sr.ReadLine());
            }
        }
    }
    private void btnOutput_Click(object sender, EventArgs e)
    {
        //Store Original Array
        string Output = "Original 'n";
        //Output Original Array
        for (int i = 0; i < Original.Length; i++)
        {
            Output = Output + Original[i] + "'n";
        }
        //Create TempArray
        int[] TempArray = new int[Original.Length];
        //Set TempArray Equal to Original Array
        for (int i = 0; i < Original.Length; i++)
        {
            TempArray[i] = Original[i];
        }
        //Duplicate Number Counter
        int Counter = 0;
        //Loop Through Entire Array
        for (int i = 0; i < TempArray.Length; i++)
        {
            for (int j = i + 1; j < TempArray.Length; j++)
            {
                //Replace Duplicate Values With '-1'
                if (TempArray[i] == TempArray[j])
                {
                    TempArray[j] = -1;
                    Counter++;
                }
            }
        }
        //Set Size of Original Array
        Original = new int[Original.Length - Counter];
        //Remove -1 Values
        //Index Counter
        int Index = 0;
//error starts
        for (int i = 0; i < TempArray.Length; i++)
        {
            if (TempArray[i] != -1)
            {
                 Original[Index] = TempArray[i];
                Index++;
            }
        }
//error ends
        //Final Output -- The New Array
        Output = Output + "Original Without Duplicates'n";
        for (int i = 0; i < Original.Length; i++)
        {
            Output = Output + Original[i] + "'n";
        }
        lblOutput.Text = Output;
        }
    }
}