Triggering event when mouse is clicked once

You could create a boolean and set it to true once the user clicks for the first time. When the user clicks another time the boolean is already true and the code gets executed (the boolean is then set to false again).

Like this:

bool clickedOnce = false

private void node_Click(object sender, EventArgs e)
{
    if (e.Button == MouseButtons.Left)
    {
        if (clickedOnce)
        {
            clickedOnce = false;
            string NodeName = treeSidebar.SelectedNode.Text;
            foreach (KeyValuePair kvp in RSS.list)
            {
                string name = kvp.Key;
                string url = kvp.Value;
                if (NodeName == name)
                {
                    MessageBox.Show(url);
                }
            }
        }
        else
        {
            clickedOnce = true;
        }
    }
}