This used to happen frequently, back in the old days- when we were often able to get 6 or more players. Not to mention the main campaign, where we had a habit of picking up strays as well as playing multiple characters each. Which was a hedge against the sometimes frequent low turnouts. It just made it easier for the DM to plan, with a consistent core known party size. NOT playing because x, y and Z didn't feel like playing?? Notta chance!!!
It was never a big deal- especially since the multiples were mostly fighter types. A few were also multi-classed, for example, my fighter-magic user-cleric. You PAID for that extra flexibility in AD&D- all XP were split evenly between all of your classes, AND each class had it's own level progression. Thus individual classes lagged WAY behind, and so did your total character level.
In more recent games, we try to ensure a good mix of capabilities/classes- but if say, 2 players want to be fighters or barbarians, for example, they can. We tend to play heavy adventure/ exploration with plenty of action and combat, so extra beef up front is always welcome. Also had a wizard AND a Sorcerer; and 2 clerics. It never caused us any problems. Everyone got their chance to contribute.
We never did have any of the internet tropes, like parties full of paladins, rogues, Rangers, or bards, lol. And the vast majority of players stuck to the core PC races: humans, elves, and dwarves especially. Drow were also popular. Nobody ever played a tiefling; dragonborn, or Gensai, that I recall- and I played the only Aasimar I ever saw. Eberron released after the group broke up, got lives, and stopped playing- so Warforged were never an option,