RichTextBox撤消添加空格

我已经为RichTextBox创建了自己的撤消系统，无论何时执行某项操作，都会将撤消操作添加到堆栈中，当您按下撤消操作时，此操作将被撤消。除了RichTextBoxes之外，这种行为完全适用于我为其实现的所有控件。我已经将系统简化为最简单的元素，无论何时按下删除，它都会将当前选定的文本及其索引添加到堆栈中，当您撤消此操作时，它会将文本放回此索引。这是删除了最简单元素的代码（就像实际读取文本文件一样）：

// Struct I use to store undo data
public struct UndoSection
{
    public string Undo;
    public int Index;

    public UndoSection(int index, string undo)
    {
        Index = index;
        Undo = undo;
    }
}

public partial class Form1 : Form
{
    // Stack for holding Undo Data
    Stack<UndoSection> UndoStack = new Stack<UndoSection>();

    // If delete is pressed, add a new UndoSection, if ctrl+z is pressed, peform undo.
    private void Form1_KeyDown(object sender, KeyEventArgs e)
    {
        if (e.Modifiers == Keys.None && e.KeyCode == Keys.Delete)
            UndoStack.Push(new UndoSection(textBox1.SelectionStart, textBox1.SelectedText));
        else if (e.Control && e.KeyCode == Keys.Z)
        {
            e.Handled = true;
            UndoMenuItem_Click(textBox1, new EventArgs());
        }
    }

    // Perform undo by setting selected text at stored index.
    private void UndoMenuItem_Click(object sender, EventArgs e)
    {
        if (UndoStack.Count > 0)
        {
                    // Save last selection for user
            int LastStart = textBox1.SelectionStart;
            int LastLength = textBox1.SelectionLength;

            UndoSection Undo = UndoStack.Pop();

            textBox1.Select(Undo.Index, 0);
            textBox1.SelectedText = Undo.Undo;

            textBox1.Select(LastStart, LastLength);
        }
    }
}

但是，如果您只选择一行中的 n，以及下面的更多文本，请执行以下操作：，然后按删除，然后撤消，它似乎会撤消此 n字符两次。