Having the same start date and end date should not be a problem, but it will require that the results have that exact DateTime - including matching hours and seconds. To solve this, you could do something like this:

.Filter(x => x.StartPublish.InRange(startDate.Date, endDate.AddDays(1).AddTicks(-1)))

This will make sure that you get all results with the current date.

You could also use MatchDay:

            if (endDate == startDate)
            {
                query = query.Filter(x => x.StartPublish.MatchDay(startDate.Year, startDate.Month, startDate.Day));
            }
            else
            {
                ...
            }