change section marker error

I give a part marker to a page

Then I want to give the part marker back to the same page

I'm getting an error

I can not change current (available) section marker

Error: This page already has another section

var w = new Window("dialog", "Section");
var panel = w.add('panel', undefined, "----");
var grup = panel.add('group', undefined, "");
grup.orientation = "row";
grup.add('statictext', undefined, "Page");
var sayfa = grup.add('edittext', [0, 0, 50, 25], "1")
grup.add('statictext', undefined, "Section Marker");
sayfa.graphics.font = "Tahoma:16";
var kisim_isaretcisi = grup.add('edittext', [0, 0, 50, 25], "A")
kisim_isaretcisi.graphics.font = "Tahoma:16";
kisim_isaretcisi.active = true;
w.add('button', undefined, 'Tamam', {name: 'OK'}).onClick = function() {
w.close(12345);
}
if(w.show() == 12345) {  
var doc = app.activeDocument;   
var ksm = kisim_isaretcisi.text;  
var syf = Number(sayfa.text);  
var addd = doc.sections.add(doc.pages.item(syf-1));
addd.marker = ksm;
}