[PinkFloydLover]

Try something like

pawn Код:

new usedSkins[300];

public OnPlayerRequestSpawn(playerid)
{
    new skin = GetPlayerSkin(playerid);
    if(usedSkins[skin])
        return 0: // return 0 to prevent them from spawning

    usedSkins[skin] = 1;
}

I wrote this out on my iPad so my apologies if there is any mistakes, you also would have to set the variable back to 0 whenever they disconnect, now that I think about it you would also have to set it back to 0 if the player changes his character a second time, a better way to do it would be to set the variable under player spawn/death

It's late now but I'll fix it up tomorrow