And what is the problem with it now? What exactly doesnt work as expected or desired?

the prob is:
if you add, for example, a firstaid-kit to a unit.
the item reproduces itself in the inventory, if someone joins the game.
so if you have 8 players and at 1st 4 ppl start the game and the other 4 joining 10min later. you have sometimes 20 firstaid-kits in your inventory...
the items doubles or tripples with every join...

so i think its a local/global-problem...

this happens with all items...

you should really take this to BIS forums, editing section, not a feedback tracker. Peeps there are very helpful and will tell you what needs to be done to avoid this. This has nothing to do with locality but with the fact that mission runs for every client joining, while you need to run it only once.

isServer is what you need

if (isServer) then {
//code here will run only once
};

I know the "if server"-solution, but i dont want to use a external script...

You can put isserver right where you put your additem command. As I said, BIS forum is for the win!