I have also encountered this bug.

FE Scenario, Map 2: I crossed the river NE of the starting town in a NW direction to the small town across the river that starts a mini quest to fight some thieves. After going through the screen for the quest, my party now consisted of 1x Main Hero and 2x each of my other two Heroes. The two dupe sets of Heroes level up independently and make the area very easy.

I've also seen this as reported by jlzignego. Seems to happen any time I load a game in map 2 I get an extra set of heros decked out in their scenario starting gear that they came into map2 with. Loading in map1 was fine. I guess I'll wait for a patch to play through the campaign. I don't want the campaign to turn into clone wars.

This just happened to me on Map 3. It actually happened after I reloaded due to the Chapter 9 bug where there aren't 3 villages to liberate. Now I have 3 sets of each hero....

Would love a dev response on this. I really, really don't want to have to re-start the campaign.

Edit: Even reloading back to a save in map 2 still produces clones of each hero, except without Dariun.

Edit 2: It should also be noted that when this happened, the upper left of the UI no longer displays icons for my party or city.

I got this just now also.

Load did not help. gonna try and see if restart helps...

It was on scenario map 3, but it seems it happens kinda random for people.

Any word on a fix for this ?