The general concept you are asking about is known as "cascading combo boxes" - this is where the values available in subsequent combo boxes are based on previous selections...
Depending on exactly what you are wanting, you could have a simple select statement for your first combo box like this:
select FIELD from TABLE order by FIELD
Then for your next combo box, have something like:
select FIELD from TABLE where FIELD <> SELECTED_VALUE_IN_COMBO1 order by FIELD
This is the general way to achieve what you are after. Have a crack at it and post again with your code if you are getting stuck on a specific part.
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