The Harlot can visit one player a night, but they cannot visit the same player two nights in a row.
In the morning, the Harlot will receive a report of each other player that "visited" their target. This may include village roles, such as the Seer, or it may involve a wolf role, such as a Bloodhound or a normal Werewolf kill.
If the werewolves target the same player as the Harlot, the Harlot will only see a single wolf visit in their morning report. This is the "killing wolf" and is the same wolf that would die to a huntsman defence.
2016-08-20: Harlot changed so it can no longer visit the same player two nights in a row