Listbox Control unable to select items with same value

I have a list box control (Multiple Selection):
 
<asp:ListBox ID="mLbMoney" runat="server" Rows="15" AutoPostBack="True"
    onselectedindexchanged="mLbMoney_SelectedIndexChanged"
    SelectionMode="Multiple"></asp:ListBox>
 
I let user insert the money they spend line by line. Something like:
1000
2000
1500
1000
 
There are two 1000, and they are different item, although appears to have the same display and value.
 
But unfortunately ASPNET list control is not able to select the second 1000. It always jump back to the 1st value after the auto postback. (I didn't implement any code in the auto postback even yet).
 
This is what ASPNET designed for? Any easy way out?
 
Thanks in advance
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