Selecting next item from a dropdown list when similar values are present and one is default

I have a dropdownlist which is bound to a datatable has 2 similar values. One is the first item which is by default selected when page loads. Other is the last item in the list. But whenever user selects the last item selectedindex always shows as 0. Though their display values are same the other coulmn values are different in the underlying datatable. So I want to get the correct index in order to pick the rest of the column values from the datatable. Please help on this.
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