Don't run the statement twice. If you ask SQLLite to add a column that is already there, then throwing an error is the expected behaviour.
If you end up with update scripts in your app like this (which is not uncommon) then the usual way to deal with it is to store a version number in your DB and apply upgrades incrementally as needed. As you haven't done that yet, you can also assume that if no version number was present then it's version 1. Just check this somewhere when your app starts up.
Pseudo code:
var version:int = getAppVersionFromDB();
if(version < 2)
{
applyV2();
}
if(version < 3)
{
applyV3();
}
... etc ....
Then just update functions similar to as you have now.
private function applyV2():void
{
... some schema changes here ...
updateSavedVersion(2);
}
You get to flesh out the functions yourself.