I am making an assumption here. But I think you get the idea. Assuming the duration field is a difference between two date fields, [From Date] and [End Date], I would have a formula checkbox in that table that says [From Date]=[End Date]. Then bring in that checkbox as a lookup field and that is the field you want. The logic here being that at the end of the day, the duration is the difference between two or more date or date-time fields and if you just want to see if the difference is zero, that means those two fields are equal. Does that help?
