Let's say a PC casts charm person on the town's sheriff. One hour later, the spell ends and the sheriff realizes he has been charmed, and is now furious and out to arrest the PC. During the confrontation, the PC casts charm person again. Does the sheriff simply forget/forgive his previous vendetta and let the PC go free? Does the knowledge of being previously charmed affect the interaction in any way? How would you play out this scenario?
Charm Person is not mind control
The description of the Charm Person spell states:
You attempt to charm a humanoid [...] If it fails the saving throw, it is charmed by you until the spell ends or until you or your companions do anything harmful to it. The charmed creature regards you as a friendly acquaintance.
Charm Person does not modify memory, neither it allows the caster to command the charmed person. It's only effects are:
The person becomes charmed:
"Charmed" is a specific game term:
A charmed creature can't attack the charmer or target the charmer with harmful abilities or magical effects. The charmer has advantage on any ability check to interact socially with the creature.
The person "regards you as a friendly acquaintance".
The latter is ambiguous and requires a DM's adjudication. I'd say it shouldn't affect the sheriff's actions much: "you're a good guy, but the law is the law". However, your mileage may vary. Choose an option which seems correct to you and stick to it in the future gameplay.