SCUM... but that is about it.
Rebels should never fight, unless their is an exposed traitor, then it would be that SOB's ship trying to get away from the rest of The Rebs.
Empire same deal as Rebs... but full on fights are not cool or fun IMO.... except for the Pirates, they fight kinda like tribes of orcs.
And yet Rebels fly training missions all the time against other Rebels. There are whole sections of the novels that revolve around it. It's the main reason I like Horton Salm - the training missions he and his Ys fly against Rogue Squadron in the X-Wing books.
Imperials have to do the same thing. And after the death of the Emperor, they are fighting each other more seriously.
Scum fight Scum all the time. Black Sun and the Hutts don't get along, for example.
Same factions "fighting" each other has plenty of justification in the background. There's no reason to dislike it.
Play fighting and training is not what is being discussed here.