Spot the difference puzzle

Hey, how's it going.

I'm having trouble creating a particular puzzle for my group.
Basically what's goingt to happen is that they are about to unknowingly intrude on a fey's home. The fey will not be hostile, but demand that they entertain it for intruding on it. It will then cast a spell on one of the players, creating several duplicates of them. The rest of the group then has to guess which one is the real one. If they guess correctly, it will just let the party go, if not... 
The thing is i'm having trouble to figure out a way for the group to determine which one is real. Obviously all of the copies would claim that they are the real ones and giving them some sort of obvious visual indicator that you can just spot with a high enough roll would be too easy and render the puzzle pointless. I want the player to be able to solve the puzzle, but for it to at least challenge them a little bit. Does anyone have a good idea what to do?

As alwys, thx in advance for taking your time to read and help out.

Perhaps a variation of a logic puzzle?

The fey casts a spell that creates a bunch of duplicates of the PC in question, but all the duplicates are different. Different skin colours, different hair colours, that sort of thing.

The fey then tells them "The oldest copy is the real copy" and gives them a bunch of clues like "the red haired one is younger than the blue-skinned one" and "the one with the tail is not the youngest."

Not quiet what i had in mind, but something worthwile to look into regardless. Thx for the tipp^^

If the players are allowed to ask questions, they might b able to come up with a way to ask a question that only the real "copy" would know - but e.g. they are only allowed to ask one question of each copy for example? (Of course the order might be important. If the real character is first in line, the others might copy the response)

Alternative, you could simply set DC for insight or investigation to spot small differences if the players are stuck or you want to resolve it by rolls instead. 

You can some variation of the Vampires do not have reflections bit.   

Put some mirrors around and have the real one have a minor difference such as different eyes or some horns, etc. but only in the mirror.

If you want to make it a challenge for your players then put out 5 pictures of the same character with slight differences. Different eye color, different hair color, different types of clothes, different weapons and see if the players can remember what their compatriot was supposed to look like. (Don't let them look at the character sheet of course). If they get stuck then allow some die rolls to see if they remember what color his eyes are. If they say that their characters should know that already then have the players all close their eyes and say the hair, eye color and current clothes being worn of the player whose character is duplicated just to see how observant the players are :)

I know, I know! Make one of the players be replaced with the fey, and everyone gets to ask one question. The fey doesn’t know anything about the character, so they have to bluff. If they guess correctly, they can go. If they’re wrong, the stolen character stays.

How long has the party been together (in game sessions) and how do they get on with each other? Do they have secrets they keep or anything like that?

The fey are tricksters, and it would be just like a fey to split the character in two, and then give them the well-known "One of these speaks only truth, the other only lies, they can only say yes or no, so which one is your guy?", and make the player roleplay both, with all the knowledge of the character, and the knowledge of which is the fake (roll randomly to determine if it's the True or the False one). The party can ask each of them questions, but only one at a time, to stop it from getting too messy.

The party will do one of three things, in all likelihood:

1.  They will immediately ask the typical question to correctly solve the riddle: "If I were to ask the other who is false, what would they say" - and then solve the riddle, to the fey's disappointment.
2. They will ask loads of questions to guess which of them speaks the truth and which speaks lies, and then work out which is the false one from that, to the fey's mild amusement.
3. They will realise that this is an opportunity to ask the character anything and have a truthful answer, and will capitalise on it, asking questions which will reveal secrets, to the fey's great amusement and delight.

The fey will then un-split the character and allow them to proceed. They will either allow them passage if they solve it the boring way, or they will offer them comfortable accommodation if they solve it the middling way, or they will offer them a full-blown party if they use the opportunity to get answers!