asp.net Modaldialog issue

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C#

ASP.NET

Javascript

I have an issue with Modaldialog,
1. I use 'window.showModalDialog' to open a modaldialog and Save Data,but when I click the button, another modaldialog open(the same as first modaldialog).
2. And in the asp.net page, there are 2 DropDownList Controls ,one of them set

AutoPostBack="True"

property, when I open the modaldialog then select a Item form the DropDownList, also a new ModalDialog popup .
3. I tried to use

window.open

, all is fine .
The ModalDialog can't Refresh ?!
I dont know why,can you help me.
My code:

function addDetails(empno, year, month) {
    var rav=window.showModalDialog("/Pages/Staff_Balance_Details_Add.aspx?empno=" + empno + "&year=" + year + "&month=" + month, '_self', "dialogWidth=600px;dialogHeight=450px;center=yes;scroll=no", 'dependent=yes, menubar=no, toolbar=no,status=no,modal=yes,scroll=no');