Whenever you have the same expense (in your example Personal Car Mileage) for the same distance/amount ($20.72) on the same day (1/23) the duplicate flag will appear as a warning. If it is a round trip, you could put the entire round trip in one line instead, but I'm not sure if that applies here based on the information.
@ngetchell I would take @JessicaL suggestion and combine the trips into a single entry. For example, you have two entries for January 23rd. Since the mileage was incurred on the same day, it can be entered as just one trip. It will actually save you a little bit of time as well. The idea behind Car Mileage is being reimbursed for a total distance driven in one day, so all the miles incurred on the same day could be entered as one expense entry.
However, if your company wants you to do it the way you have shown, by entering each leg of your trip as a separate entry, then you will just have to be okay with the yellow exception. The yellow exception is just a reminder or warning, you can still submit the report.