如何禁止用户按住某个键后在文本输入框中连续输入相同的字母

private char last1 = '/0';//存储倒数第一个键的值
private char last2 = '/0';//存储倒数第二个键的值
private void txtTest_KeyPress(object sender, KeyPressEventArgs e)
{

//如果按下的是退格键，重置变量
if (e.KeyChar == Convert.ToChar(Keys.Back))
{
last1 = '/0';
last2 = '/0';
return;
}
if (last1 == '/0')
{
last1 = e.KeyChar;
}
else if (last2 == '/0')
{
last2 = last1;
}
else
{
last2 = last1;
last1 = e.KeyChar;
}
if (last2 == last1)
e.Handled = true;
}

//释放按键后，重置变量

private void txtTest_KeyUp(object sender, KeyEventArgs e)
{
last1 = '/0';
last2 = '/0';
}