Top N Not Displaying correcty (Top 5) When data items have the same metric

I have a Top 5 Report Based on Duration of Job against a Property number

Property Number Top N (5) By Duration

This is great when they all have different Durations But it many cases this isnt the case

Property Number     Duration

1                                500

2                                400

3                                 300

4                                300

5                                300

6                                300

7                                300

8                                300

9                                300

10                                300

So this would show me 10 rather than 5 And I cant at the minute figure out how to deal with this scenario

For ease I think I would use Duration and then just select  the first property IDs in the list where all the durations are the same but Im not sure how to do this?