[DanishHaq]

pawn Код:

format(string, sizeof(string), "Accounts/%s.ini", sendername);
                if(fexist(string))
                {
                    SendClientMessage(playerid, COLOR_YELLOW, "That name is already taken, please choose a different one.");
                    return 1;
                }

This code is checking if the player's account is there in the Accounts folder, not if the player is connected, that's another slight issue I found with this script. So if it exists in the Accounts folder, it'll say that the name is taken.