While editing a line item, you will also be able to choose if you want to activate the Frequency Capping.
Frequency Capping assures that the same line item will not be delivered to the same download more than once. If the box is checked, for a single download there will be just 1 impression for line item. You will not listen 2 times to the same ad in one download.
If you have more than one roll targeted, the ad will go on the first roll available.
This mean that even a line item targeting all the slots (preroll/midroll/postroll) cannot be added twice (for example as a preroll and as a midroll) in the same download.