Anytime you import a property using the "Import List" button on the campaings screen, Pebble will check to see if that property already exists in your Pebble account. It does this by checking the property's county and APN (Assessor's Parcel Number).
Let's say you have an existing property in Monterey County, CA with the APN "1234". If you upload a new list for Monterey County and one of your rows has the APN 1234, Pebble will not create a new property. Instead, it will simply update the existing property with any new data from the newly imported list.