If I add a unit to a group that already contains the unit then nothing happens,right?

Right?

Wrong! you break the internet if this happens, ill explain for you:

if this map is online and there are 2 instances of the same thing simultaniously then it will automaticly create a second object, it is assuming that the two are different, however they will be linked due to blizzard parameters, in this sense the data packets used to form the virtual network between the players will be breached and cause global data leaks, this will square with each other player added, so in a 4 player map your looking at the leak being 16 times worse then a single player map, so you can see how quickly it can escalate.

the data packets then cause a domino effect, as they themselves are the SAME packets, however they are multi instanced so that the same effect happens AGAIN!!! and because of the squaring effect of multiple players this can quickly lead to a mass overload of the global servers.

so you realy should add a condition: "If (Unit) is in (group) Equal to False" to prevent this from happening.

AKA: no.

Gorman I understand you were being sarcastic however knowing blizzard something along those lines would be entirely plausible...

Hm interesting... But wat if i wanna loop trough a unit twice mor triple, inside a unit group without using loop

yeah, i was actualy thinking it was the knid of thing blizzard mite do aswell =P, but i dont think there are any implications of doing it, i wouldnt worry.

so conclusion is... Is it a serious problem if i add a unit twice or more to a group?

probably not might be worth testing adding a unit a few hundred times to see if there are some wierd problems tho :/