* Characters: If you're talking about fictional characters in a show, book, or movie, they are likely played by different actors.
* Real people: If you're talking about real people, they could be the same person, different people, or nicknames for the same person.
* A misunderstanding: You might be thinking of two different characters, or maybe a name is misspelled.
To help me understand your question better, please provide some more information:
* What is the context of your question? (Are you talking about a specific show, movie, or book?)
* Can you provide any other details about Derf and Fred? (Like their full names, what they are known for, etc.)
Once I have more information, I can try to answer your question more accurately.