custom city.

On a form with a dropdown combo with the control source being a city field in a record. The row source is SQL and I use
"Select name from city order by name in to cursor temp" to fill the combo. They also may type in a city not on the list. What happens is when I bring up the form on a record that had a city not on the list it blanks out the combo instead of showing the value in the city field of the current record. Appearantly if it isn't in the combo's item list it shows blank? Whats up? Is there a workaround? I have tried manually populating the combo with additem and then also adding the custom city but it is much slower and how do i know that the custom item hasn't already been added to the list and add it again without scanning the entire item list (about 400 cities). Anybody got any ideas?