[DanishHaq]

Untested, may not work at all:

pawn Код:

new pOriginalName[MAX_PLAYERS][MAX_PLAYER_NAME];
forward KickTimer(playerid);

public OnPlayerConnect(playerid)
{
    new name[MAX_PLAYER_NAME] = GetPlayerName(playerid, name, sizeof(name));
    for(new i = 0; i < MAX_PLAYERS; i ++)
    {
        if(strcmp(pOriginalName[i], name, true) == 0)
        {
            SendClientMessage(playerid, -1, "The name you logged in with is already connected under a clan owner's name.");
            SetTimerEx("KickTimer", 1000, false, "i", playerid);
            return 1;
        }
    }
    pOriginalName[playerid] = name;
    return 1;
}

public KickTimer(playerid)
{
    Kick(playerid);
    return 1;
}

public OnPlayerDisconnect(playerid, reason)
{
    strmid(pOriginalName[playerid], "", 0, strlen(""));
    return 1;
}

It doesn't require MySQL at all.