Origin is set up like that by default. If you have an 8-hour trip, your calendar automatically prevents you from being scheduled for any additional trips.
To make sure of this, you can add a 60-minute buffer time to the 8-hour listings. Here's how to do that:
Go to Listings
Click on the title of the listing you want to edit to expand details (you can use the search bar to narrow it down more easily)
Click the gear icon
Scroll down to Buffer Time (in minutes)
Enter "60" in the field
Click Next to save