You know, this got me thinking. Heroes have already shown there are multiple versions of Alfonse and Sharena, basically suggesting the fact that you can summon multiples of a same character is actually somewhat canon to that game and not purely a weird gameplay mechanic that gets ignored by the story.
That being the case, I wonder if they've set it up sort of like this - every person's playthrough of a fire emblem game is considered its own parallel universe. This is why multiple versions of a character can exist. Expanding on that, maybe you're right? What if summoning a hero is related to the permadeath mechanic in the sense that these are heroes a player lost in their game so from a story perspective, they will not be "missed" in their own world.
Or maybe not, but it was just an interesting idea :P