A joke usually isn't as funny the second, third, or tenth time you hear it. After a while you tend to say to yourself, "I've heard this before. I know how it is going to turn out."
Funny, your kid says the same thing when you repeat yourself when making a request or enforcing a rule. While some repetition is necessary to help in learning, most parents make the mistake of repeating themselves trying to get a point across to their kid.
Therein lies the problem. Repeated reminders in the same conversation are not helpful. Repeated reminders over a period of time in different circumstances is helpful. The first instance is nagging. The second is teaching.
Are you nagging or teaching your kid?