It's probably a field definition problem, not a listbox problem. You could manipulate the listbox, but that's not really the right solution.
My guess is that you're doing something like this:
date(MyDate,'MMM') as Month
The problem with that is that all dates are still stored as dates, regardless of how you display it. The 'MMM' is only telling it how you want to see it on the screen. So September 1, 2008 will still be different than September 2, 2008 will still be different than September 2, 2009. As a result, you'll see Sep repeated in your list box for every date in September of any year.
If you want all dates in September of any year to be equivalent, use this:
month(MyDate) as Month
The month() function drops the date information, and stores only an integer month (1-12) with a display format of MMM.
If you want all dates in September of EACH year to be equivalent, but different from September in other years, use this:
date(monthstart(MyDate),'MMM YY') as Month
The monthstart() function forces all dates in the month to have the same value.
