[regalis]

I think you should use a bool with that you can trace if the player have already a nade ;)

try this:

public cmd_ammoh (id)
{
    if ( !get_pcvar_num ( g_switch ) )
        return PLUGIN_HANDLED
    
    if ( !get_pcvar_num ( g_grenade ) )  
    {
        client_print( id , print_chat , "[AmxX] Currently unable to purchase grenades." )
    }
    
    if ( !is_user_alive (id) || !is_user_connected (id) ) 
    {  
        client_print (id , print_chat , "[AmxX] Unable to buy HE-Grenade due to your status (Dead)." )  
    }
    
    new money = cs_get_user_money (id)
    new multi = get_pcvar_num ( g_multi )
    new bool:haveOne

    new weapons[32] 
    new num, weapon
    
    get_user_weapons(id, weapons, num)
    for(new i = 0; i < num; i++)
    {
        weapon = weapons[i];
        
        if ( weapon == CSW_HEGRENADE )
        {
            client_print ( id , print_chat , "[AmxX] You already own one," )
            haveOne = true
        }
    }
    if ((money < ( multi * g_he_nade )) && (!haveOne))
    {
        client_print ( id , print_chat , "[AmxX] You do not have enough money." )
    }
    else if(!haveOne)
    {
        give_item (id , "weapon_hegrenade" )
        cs_set_user_money ( id , money - ( multi * g_he_nade ) )
        client_print ( id , print_chat ,  "[AmxX] Thank you for using Emergency Ammo!" )
    }
    
    return PLUGIN_HANDLED
}