Obviously just keep him! Part of the appeal is the attachment you get to your villagers. If a villager sticks, then it is meant to be! I have had villagers that I grew attached to ask me if it was okay to move a couple of times in a just a few weeks (or 'move bubble cycles', if you like), and that's often a reason I might let them move after all. Then it was 'meant' to be, whatever that means in a virtual world, haha!
I would keep him! I'm the same way. I have the amiibo cards of most of my villagers, but when I think about letting them move away and then getting them back, I know that they won't be the same as the one I let go.
i have both of them on my own island and it’s honestly a blessing. if you have room to keep both of them, i don’t see why you can’t tbh. come on it’s the lip balm duo! /also maybe bc i’m a bit biased towards Marshal lol.