DropDownList - To Tell The Computer Existing Id?

Right now, I am struggling with this ASP.NET problem. I want the computer to display a label that says, "ID Already exists -- must select a different value" when the computer has a matching (or the same) number in the dropdown list box. When I add a NEW ID, this is when the process happens.If the New ID does not = to the values in the ID Number, then when I press 'Add New Item', the red label should display:ID Already exists. Must select a different value. Right now, my code for the add button event is:
If ddlItems.Text = "Add New Item" Then
Dim item As New Item
With item

[Code]....

What can I do to add the text below and read all IDNumbers to verify that it is not chosen already?