[ericthered]

Extra attack appears in the worked Genre sets a number of times:

DF has it as an option for the swashbuckler and as a chi ability. Some of its monsters also have it, including the Peshkali, which has extra attack 5.

Monster Hunters offers Extra attack to inhumans, experiments, and warriors. Its list of advantages indicates a single level is appropriate rather than many (experiments may exceed this limit, but only as part of a power set). Its enemies book hands out the advantage with Gusto: Strong Demons, Demon Lords, Albino Alligators, Mature and tougher vampires and Angels all have various levels of it.

After the End give the Hulk Extra attack, as does the "Fast" Upgrade, but the book carefully rules that you may only take one level total.

Action generally makes extra attack part of martial arts, requiring trained by a master or weapon master in order to take it, but allows a perk to waive that requirement.

All of these focus on extra attack as being useful for close quarters combat, rather than ranged combat. Extra attack technically can be used for ranged combat, but I don't see it much, as effective use requires other advantages.