Npc are not all that different from player characters in my opinion. They are introduced in the game by players,the same as player characters. The only difference from player character I see is that they can be used by multiple players.
My question is: can exclusive rights to certain npcs be set by either a player or gm?
You know only the following player or players are allowed to roleplay this Npc. Can gm reserve exclusive rights to roleplaying a certain npc, or simply establish a guideline on how the npc should be roleplayed?

The GM has the ability to set any rules for his/her game that she/she desires. If the GM wishes to retain exclusive control of NPCs for the sake of controlling the story, that is entirely at their discretion. As long as the GM doesn't have a problem with it, players can also create NPCs and then control them exclusively or share them as they see if. It's about all about what the particular RP wants and the best way to accomplish goals.

However, it's when the player characters take excessive liberties with the ability to create NPCs that problems often arise. This isn't to say that characters can't have servants, it's that they shouldn't be able to call on these people to help them with any imaginable situation without a damn good reason.

I agree with Jag. Of course a GM can. A GM can do anything, and to keep exclusive control of NPCs or at least certain NPCs is nothing out of the ordinary. It all depends on what the GM rules appropriate for the game. Then, in some cases, it's secondarily up to the players whether they would like to share their NPCs or control them exclusively. I would say never assume that a character someone else created -- PC or NPC -- is up for grabs for you to control unless it's in the rules that that's how it works or you have an understanding with the player because these standards do vary from game to game and player to player.

I've never actually experienced that phenomenon, QB. I should probably count myself lucky. xD In that case, it's up to the GM to put his or her foot down and call the player out for unrealistic, unfair behavior. Using a plot device to solve any and all of a character's problems regardless of circumstance is a form of godmoding.

Marionette: Watch Barney get into a bar fight sometime and you'll know EXACTLY what I mean. He's by no means the only person to do it--indeed, much of the "NRP" shenanigans is about that. He just does it more.

I think there is a difference between "named" and "unnamed" NPC's.

I.e.

Player1: *Le bartender serves me a drink* Hallow!
Player2: *le bartender serves me a drink too!* Hulluahz!

A named PC could be a plot npc played by the GM, I have also seen personal npcs which I think do belong to particular players.

Player 1: This is my sister Lacey.
Player 2: *Your sister smiles and winks at me, she takes me upstairs to her room!* BAI!
Player 1: NO WAI! My sister would never do that! She is a nun!

Perhaps I am stating the obvious. If a more prominent npc is shared it would likely create personality inconsistencies. If it doesn't matter then... it doesn't matter. It can always be discussed OOC if there is a question.