// Look through all open forms to see if there is already a Form2 open.
// If it is, Close it. This prevents the user from (purposely) having 5 of the same forms open.
// If you try to cycle through open forms after closing one, it crashes, 
//     because it edits the list of forms while cycling through it
// This is why it is converted into a list of forms first.
foreach (Form frm in Application.OpenForms.Cast<Form>().ToList())
{
    if (frm.Name == "Form2")
    {
        frm.Close();
    }
}