If a player decides that he's the only one who gets to have a certain ability, and gets upset that other players are 'stepping on your toes' if they take similar abilities, then they are in practice trying to make a rule for the table. They generally won't come out and say that they want to make it a rule, but that's what they're actually trying to do.
Yes, and I'm saying that if a person, especially one who doesn't even understand the game, is the kind of person that decides that if their character does something they now 'own' the concept and anyone who does something similar is 'stealing' from them, then they're the kind of person I don't want to play with. If the only way you can have fun is by restricting the choices of other players for their characters, you're not the kind of person I want to try to play with.
No, I want to avoid playing with them because they are arrogant and entitled enough to decide that they own basic game concepts (before they even understand the game!), think that they get to decide what other players are allowed to do with their characters, and base their fun off of limiting what other people's characters can do. The kind of person who is playing a warlock, sees that someone else has some warlock spells, and decides that they're now a victim of theft is just not going to be pleasant to play with, and down the road is going to become like the example someone else gave where no one but That Guy gets to play a wizard because he'll make their experience unpleasant if they 'step on his toes'. This is all about a person's attitude towards other players, it has nothing to do with knowing the game system, and in reality is NOT the kind of thing that brand new people do, but they kind of thing that people with some system knowledge and a bad attitude do.
This will likely backfire, but lets switch gears for a second. Imagine you have little to no idea what DnD is, except that it is a board game. I know it isn't, but people learning about the game for the first time probably don't have an understanding of an RPG, they'll default to things they understand first.
So, they think about other games they have played. Comparing it to these other games til they figure out what it is like. This is normal human behavior, and it works 90% of the time, but it does lead to some growing pains.
How many people can play the top hat in Monopoly? Well, one. During that particular game.
How many people can be the Blue player in Sorry? Well, one.
How many people can be the tank during a WoW raid? I admit to not being a very knowledgeable person in MMO's but from everything I've seen, an effective group has one.
So how many people can play the warlock in DnD? Well, going off the basic pattern recognition, a person with no prior knowledge could easily assume one. And DnD is often portrayed with the classic 4-man party. Even the art whenever it shows a "group of adventurers" shows a group with a highly diverse skill set.
Now, a new player is wrong when they assume only one person can play the warlock during the DnD game, but they are just confused, not malicious. Tabletop RPGs are like no other gaming in the world, they are incredibly complicated, and in trying to navigate that complexity new people seek out a role in the party. "I'm the one who fights on the frontline" "I'm the one who sneaks" "I'm the one who heals".
And when that role is threatened, they can feel bad about it. IF they aren't the one who does the thing... then did they do it wrong? Do the other person do it wrong?
Again, as experienced players, we know that isn't the case. Nobody has done anything wrong. But, the new player doesn't know that, so they turn to the most experienced player, or the person who seems like they have the most experience and asks what is up. That's generally the DM. They could also turn to the person who brought them to the game, and then that person may talk to the DM.