Just keep track of the last form you created in a variable:
private Form lastForm;
private void showForm(Form frm) {
frm.FormClosed += (sender, ea) => {
if (object.ReferenceEquals(lastForm, sender)) lastForm = null;
};
frm.Show();
if (lastForm != null) lastForm.Close();
lastForm = frm;
}
And use showForm() to display your forms:
private void addCampusToolStripMenuItem_Click(object sender, EventArgs e)
{
showForm(new if_add_campus());
}
Not tested, should be close.