If the values you are adding to the combobox are coming from a database, the easiest solution would be to only query the database for unique values. You take care if that in the SQL statement you use. You will probably use the DISTINCT keyword in the SQL statement. For example, if you are popluating a combobox with States and the table you are pulling the States from contains addresses of all your customers, the table would contain (among other things) fields like:
[Address1], [Address2], [City], [State], [Zip]
Now, the SQL statement you would use to select unique states from that list would be
"SELECT DISTINCT City FROM YourTableName ORDER BY City"
The distinct keyword decides what is distinct based upon the fields that are in the SELECT statement. For example, if you had a table like this
addr1 city
1 Chicago
2 Chicago
and you used this SQL statement "SELECT DISTINCT * FROM Table1" or "SELECT DISTINCT addr1, city FROM Table1" you would get a recordset back with 2 rows, one for each addr (since the combination of 1 and Chicago is different then the combination of 2 and Chicago). If you used the statement "SELECT DISTINCT city from Table1" you would get a recordset with one row in it, since there is only one unique city in the table.
Now, since all you are pulling from the database is unique data, you do not need to write any of your own deduplication code. This should be faster as you are not adding data to the combobox that will only be removed from it later.