The Star Showdown

The Star Showdown is an unmarked quest in Fallout: New Vegas.

Walkthrough
Located just west of Nipton, following the road that heads west towards the NCR Mojave Outpost, you can see two non-hostile non-player characters shooting each other. The two non-player characters are named Tomas and Jacklyn and will continue to fight each other until one of them is dead. They cannot be interacted with until they have finished fighting, however, Jacklyn will shout out to the player for help. You can kill either character at this point without affecting Karma.

Once either Tomas or Jacklyn has been killed, the other non-player character will approach the Courier and initiate conversation.
 * If Tomas is the survivor, he will inform you that Jacklyn was his traveling companion he met a few days earlier. He claims they were getting along, even flirting, when she turned her gun on him and demanded his caps. When he gave her his money, she stated she wanted his necklace made of Sunset Sarsaparilla star bottle caps.
 * If Jacklyn is the survivor, she may first run over to Tomas's corpse and interact with it before running over to the Courier. She will claim that he was insane and wanted stars.

Tomas
If the player first kills Jacklyn, Tomas will approach the player and initiate conversation. Through certain dialogue, you can learn about what happened and achieve one of the following outcomes:
 * You can pass a Speech check of 50 to convince him that his 'lucky necklace' is dangerous and that he should give it to you.
 * If you demand the necklace (or fail the Speech challenge), he will become hostile and attack the player.
 * Other dialogue options will end the conversation and Tomas will leave and remain friendly.

You can kill or pickpocket Tomas to claim his seven Sunset Sarsaparilla star bottle caps for negative Karma.

Jacklyn
If the player first kills Tomas, Jacklyn will approach the player after interacting with Tomas' body and initiate conversation. Through certain dialogue, she will give you her description of events and achieve one of the following outcomes:
 * If you explain that Tomas was after Sunset Sarsaparilla star bottle caps and end the dialogue, she will remain friendly.
 * If you confront her about what she stole from Tomas's body and pass an Intelligence check of 7, you can trick her into revealing that she has stolen Tomas' 'lucky necklace' of seven Sunset Sarsaparilla star bottle caps. She will tell the player she 'can't leave any witnesses' and will immediately become hostile.
 * If you reveal that you have Sunset Sarsaparilla star bottle caps, after the dialogue closes, she will remain friendly for a short period before becoming hostile and attacking the player.
 * All other dialogue options will end the conversation and Jacklyn will remain friendly.
 * After dialogue, if you can find his body, Tomas will have his journal on him that is normally found on death. This reveals that Jacklyn is lying about being ambushed and you may confront her if you can catch up.

You can choose to pickpocket Jacklyn for the Sunset Sarsaparilla star bottle caps and 10mm round ammunition incurring bad Karma. You can kill her at any time with no Karma loss; even after the gunfight or before you talk to her. After she loots Tomas' body, she will have 9 Sunset Sarsaparilla star bottle caps, otherwise she will have only 2, the other 7 will still be with Tomas.

Bugs

 * Jacklyn's body may not be present if Tomas runs up to the Courier, it disappears from the gameworld.
 * Sometimes the gunfight doesn't happen.
 * Jacklyn's body can spawn in Nipton if Tomas runs up to the Courier (witnessed stuck in the floor and writhing about due to faulty collision, but was still lootable).
 * When walking on the road from Nipton to the Pit Stop, Tomas will be walking away and Jacklyn's body is nowhere to be found. Oddly enough, killing Tomas rewards you with 0 caps as his necklace will have mysteriously disappeared.