To reproduce:
Scenario 1:
- Expand RadDropDownList.
- Type in a value and then select desired value with mouse.
- You will see that the KeyDown event is triggered twice.
Scenario 2:
- Expand RadDropDownList.
- Type in a value and then press Tab or Enter key.
- You will see that the KeyDown event is triggered three times.

Workaround:
 radDropDownList1.DropDownListElement.TextBox.TextBoxItem.KeyDown += TextBoxItem_KeyDown;