[SystemX]

Well, You can do it like-

PHP код:

new oldName[MAX_PLAYERS][MAX_PLAYER_NAME];
//under the /aduty command
new  name[MAX_PLAYER_NAME];
GetPlayerName(playerid, name);
format(oldName[playerid],MAX_PLAYER_NAME,name);
format(name,MAX_PLAYER_NAME,"[ONDUTY]%s",oldName[playerid]);
SetPlayerName(playerid, name);
//And remember to put under onplayerdisconnect and /offduty
SetPlayerName(playerid, oldName[playerid]); 


That may work, not sure though didn't tested.