Aware & Unaware?

[Reffy]

I see that more people agree it would not be a surprise round.

Let me give you more details.
The characters play Chaotic Neutral...which is why they act like that.
When they play good characters, they don't do such things, though it seems they are always chaotic. No discipline at all.
But hey, it's about having fun. I don't mind as a DM.

The characters are in a dungeon. They don't know how they got here. They've lost all their equipment and need to survive as a team, or perish.
(Based on Legend of Grimrock)

Because the dungeon is brutal, they always walk with their weapons equipped. I think I would have done the same thing.
Also, they are not used to unequip their weapons. But I will remind them the next time we play.

The eventually met a doppleganger, who took the identity of one of the players. They were standing next to the doppleganger, with their weapons equipped.
The players acted smart, they asked questions to the doppleganger and the player who's identity has been copied.
However, a doppleganger can read minds (unless the subject makes a will save).

The questions didn't help, and suddenly, one of the players attacked the doppleganger.
The other players agreed during table talk at that point, and claimed they could also attack, before the doppleganger could do even a thing.

Anyway, it turns out that they killed the wrong guy. The doppleganger was with them the entire time. They killed their own guy. (Serves them right in my opinion )

Conclusion
So they didn't plan ahead, and just table talk as in game agreements. I will tell them that this is not how it works.

I remember when we played 4.0. And my character (not npc) had a discussion with another PC. The guy got angry about something and attacked me.
The DM gave him an insane surprise round. He moved and then used a daily and an encounter power (action point).
Then we had to roll initiative. He rolled higher, so he could attack again.

It's funny, but also sad.

I will keep the bluff check in order to surprise someone in mind. But for now, just rolling initiative will suffice.

 

[N'raac]

Within that setting, I'd say most potential targets would be expecting an attack. Even potential allies are probably, like the PC's, walking around with weapons at the ready and suspicious of hostile intent, so they will be wary and cannot be surprised.

The target PC, however, may be a special case. He probably wasn't expecting the attack, and may have been surprised. I agree with the opposed Bluff check to pull off the surprise action - it should not be automatic. It doesn't sound like the characters are subtle. If he is trying to conceal his intentions, that works both ways. The other PC's would also need a successful Sense Motive check to figure out their teammate is going to attack in time to also benefit from the surprise.

If that's the rule established, however, the PC's should understand that this works both ways. NPC's can also Bluff to achieve surprise if they decide to attack. I would expect others in that environment to be wary and have weapons at the ready - in fact, NOT doing so might be more suspicious. They can't be the only chaotics in the dungeon.

Finally, I'm with the "randomly attacking people you are talking with" is not just Chaotic, but also Evil, camp. In the specific case, they had a reason for attacking - belief the target was a dangerous enemy. Maybe this isn't a general approach.

Finally, you may want to point out to them that, if we leave matters as this one instance was played (ie they all got a surprise round) would stand as a precedent that an adversary conversing with the PC's before attacking also gets a full surprise round for himself and all his allies against the PC's. We've even established it works against PC's, since it was a PC who was surprised in that encounter.