Concept: During the many, many, many years Merlin and Leon had spent waiting for his royal pratness to return, Merlin's magic — still eager to help and fulfill his every need — had created physical images of Arthur and all the others every time it felt his or Leon's hearts fill with longing and despair.
The figures were not only palpable but could also speak and move and talk on their own, all based on Merlin's memories of his friends.
It happens enough times for them to get used to it.
Somewhere through the centuries, both Merlin and Leon gave up waiting, convinced that neither Arthur nor the rest would ever return.
A good amount of years pass after they giving up so that Leon and Merlin had fully accepted that Arthur's not coming and are okay with it.
But Arthur and the rest do come back, and now they have to follow these two stubborn immortals around trying to prove they are real.
Arthur puts himself in front of him, grabbing Merlin's shoulders to make him stop walking and looking at his eyes with reassurance. "Merlin, it's real, it's me."
Merlin, with two bags of groceries in each hand, annoyed by the insistence of this clone: "I think you are the worst one so far." He extricates himself from not-Arthur's hands and walk back to his car, putting the groceries away before entering it and leaving the supermarket.
Arthur stands there like a fool, mouth open and hands still kind of raised where he held Merlin. Shocked still and confused by the idiot's reaction.
Merlin and Leon: talking about the others out loud and unashamed, discussing how to get rid of them
Arthur: "Excuse me, we are right here!"
Merlin and Leon: Look at each other.
Leon: "Of course you are, buddy."
Gwaine: Follows Merlin to the bathroom when he goes to shower.
Merlin: Rolls his eyes and doesn't stop him
Arthur: Pulls Gwaine back by the ear.
Arthur: "I've loved you for a very long time, Merlin."
Merlin with wide eyes: "Holy shit"
Arthur: looks at him with hope
Merlin: pulls his phone from his pocket and opens Grindr, "Jesus fuck, I need to get laid."
I don't know, I just think it could be funny.