Asp - Duplicate Items In Dropdownlist

I'm designing a web page with two dropdownlists, one for the Make of a car, the other for the Model, both bound to a database with separate SQLDataSources, and using a distinct statement. I add "All" at the top of both by setting appendDataBoundItems = true and adding items named all. Then when I fill the Make with a querystring all the model items get added twice (but only the databound items).Here's my code:

<asp:DropDownList ID="DropDownMake" runat="server"
DataSourceID="SqlMakes" DataTextField="Make" DataValueField="Make"
AppendDataBoundItems="True" EnableViewState="False" AutoPostBack="True">
<asp:ListItem Selected="True" Value="All">All</asp:ListItem>
</asp:DropDownList>
<asp:DropDownList ID="DropDownModel" runat="server"
AppendDataBoundItems="True" AutoPostBack="True" DataSourceID="SqlModels"
DataTextField="Model" DataValueField="Model" EnableViewState="False">
<asp:ListItem>All</asp:ListItem>
</asp:DropDownList>
<asp:SqlDataSource ID="SqlMakes" runat="server"
ConnectionString="<%$ ConnectionStrings:ConnectionString %>"
SelectCommand="SELECT DISTINCT [Make] FROM [Parts]">
</asp:SqlDataSource>
<asp:SqlDataSource ID="SqlModels" runat="server"
ConnectionString="<%$ ConnectionStrings:ConnectionString %>"
SelectCommand="SELECT DISTINCT [Model] FROM [Parts] WHERE ([Make] = @Make)">
<SelectParameters>
<asp:ControlParameter ControlID="DropDownMake" Name="Make"
PropertyName="SelectedValue" />
</SelectParameters>
</asp:SqlDataSource>
'And in the VB file:
Protected Sub Page_Init(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Init
DropDownMake.SelectedValue = Request.QueryString("Make")
DropDownModel.SelectedValue = Request.QueryString("Model")
End Sub

If I remove the line "DropDownMake.SelectedValue = Request.QueryString("Make")" it doesnt produce duplicates anymore. What is going on?? I swear I've spent more time developing entire iphone app.