private static bool loginShowing = false;
private static readonly object loginLock = new object(); // 静态锁对象

public static bool? ShowLogin()
{
    lock (loginLock) // 获取锁
    {
        if (loginShowing)
            return false; // 或者 null，取决于你想如何表示“未打开新窗口”
        loginShowing = true;
    } // 释放锁

    try
    {
        var wndLogin = new LoginWindow();
        return wndLogin.ShowDialog();
    }
    finally
    {
        lock (loginLock)
        {
            loginShowing = false;
        }
    }
}

private static SemaphoreSlim loginSemaphore = new SemaphoreSlim(1, 1);

public static bool? ShowLogin()
{
    if (!loginSemaphore.Wait(100))
    {
        TipHelper.ShowTip("请验证密码！");
        return false;
    }

    try
    {
        var wndLogin = new LoginWindow();
        return wndLogin.ShowDialog();
    }
    finally
    {
        loginSemaphore.Release();
    }
}