A trip_id must be unique, meaning there cannot be two rows in the CSV file with the same trip_id.
If the same trip operates on multiple services (For example, a northbound trip on route 1 at 8:30 AM runs on weekdays, saturdays and sundays), there are two ways to represent this:
1. Have multiple trips, one for each service, each with a unique trip_id
2. Have only one trip and create a calendar that specifically represents the operating days of this trip.
If you choose to go ahead with the second option and use the block_id field, be sure that the blocks are the same for the different service days.
Hope this helps!