It is possible through clever placement of land tiles for there to be more than one thief on a road, more than one knight in a city, and more than one farmer on a field.
Two meeple can be placed separately on unoccupied features and then later joined by a new tile. In this way, both meeple share the same feature and are scored accordingly. These is no limit to how many meeple can be on the same feature when joined in this way.