so just to be clear, Do the units of the various sets overlap? is there a chance for example that I get a booster for early war and i get a unit that is also in another booster pack like the desert one of the 39-45 one?
Yes you are correct. For example the Morris Armored car is in both Early war and NA. You can review sets here: http://www.tintirinti.com/search_AAM.aspx