combobox validation for duplicate items

Question

I have  one combobox feild  with  unique key... i want to validate items of the combobox  

like i have one feild which name is Lotno  it has 3 value W101,W102,W103, now i want that my value will not be repate.

that means no duplication. than what i can do?

please reply me immediatly.

one thing is that i have add the item in combobox at run time.

                                                                            thank you