How to select multiple items in a drop-down list

JLSantiago Not knowing what your application is, I find myself wondering:

1. How common is it that your users need more than one state? (my wife and I have residences in two states, so I can see how it could arise)...
2. On those occasions when they DO (for whatever reason) need to enter two or more, would that kind of entry mess up sorting or filtering of the entire list for the sake of those few? (i.e., how do you use the entries in this field in your own analyses or services?)

• It occurs to me that, especially if it's relatively rare that this occasion arises, why not have the 51st answer on your drop down list "More than One State" and provide a comments field for use in that case?
• If it's more common, I think you'd be better served by headings like "Primary State"  "Secondary State" or the like.

I offer these thoughts solely for the purpose of what we database designers call "data integrity," the idea of which is that you want entries in any given field, to be consistent: the answer to the question, or the single piece of info that fits. When we start making exceptions, it inevitably creates unanticipated problems at some point down the road. Obviously, this all depends on how you're using that item of info; if it's purely curiosity, that's less significant than if, say, tax tables depend on the answer.