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First thing first: identify whether this is an IC or OOC conflict
In some parties, the PCs fight but the players are having fun. In others, the IC fights are a result of OOC tension. In yet others, it's because the players genuinely don't know what makes the best gaming. Solutions depend on these parameters. Once you've identified the source of the problem, you can choose an appropriate solution. Here are some example parameters and likely (but not guaranteed) solutions for different sources of the problem:
Players have fun playing conflicts to the hilt
If all the players are happy with a cutthroat, treacherous party and are having fun, it may be the best to direct them to games that support that. For example, Paranoia is famous for having high lethality, constant backstabbing etc., and treating it with a laugh.
Players like conflict, but don't want to lose their PCs
If this is the case, consider discussing it with the whole party, and coming up with a way to reduce lethality and/or similar long-term consequences. Perhaps suggest them to play out such conflicts as fistfights, not shootouts/swordfights, and declare that damage sustained in such conflicts heals faster and cannot cause a death (this isn't realistic, and many systems don't support such distinctions, but it's a viable houserule for the sake of fun).
Players think this makes sense IC, but don't like the outcome
For example, everyone has a stubborn, grudge-holding character, and so feels that the only right way to react to not getting one's way is to avenge that disproportionately later. If so, talk it out OOC, and ask players to make character parties that are more internally friendly: a party of long-term allies who met long ago and not yesterday in a space tavern, a dynasty of loyal treasure-hunters etc. This may require either adjusting the personalities of characters, filling in details that weren't specified so far, justifying a personality change as the game goes on, or even making a whole new party.
There is a hidden or overt OOC conflict between Players
Talk it out OOC and find out more. If the two players totally hate each other, at best you can hope to enforce an ultimatum against both (or more) of them that forbids PvP . . . or exile both (or more). Or maybe there's only one problem player, in which case that's who you will have to exile sooner or later (or suffer the consequences of inaction). Or maybe there was just a misunderstanding, and clearing that up will fix your problem.
It's very important to do your best to actually collectively solve this problem. Trying to contain or hide it will just result in the bottled-up conflict surging out in greater force later. So while radical measures may seem overkill, they should still be kept on the table to prevent an even worse outcome.
This can be highly unpleasant when GMing for people all of whom you deem a friend, but the reality is that often, a GM is friends with both A and B, but A and B dislike each other and when the three come together, nobody has fun as a result of said hatred. In a milder case, it may be that A and B can get along with some topic, but burst into conflict when some other topic comes up; in that case, the latter topic should be avoided.
If the whole of RPGaming is such an unavoidably inflammable topic, well, there's not much that can be done without switching to another joint hobby. However, perhaps only some subset of gaming topics are inflammable - for example, only splitting the lot acts as a catalyst for such grudges. In that case, you can try to plan your campaign around it. If looting is the catalyst, try running a campaign where looting never comes up or is handled abstractly. If sparing or killing captives is a common topic of conflicts, then try to run a campaign where most opponents are droids, or where a commanding officer tends to sweep in after the enemies surrender and takes the decision out of the PCs' hands. If the problem comes from conflicting interests in realm management, run a more boots-on-the-ground campaign. And so on.