You are not usually allowed any extra weight on the way back so should really make allowances for that when you pack initially. However, remember that things like shampoo, sun tan lotion, toothpaste etc are quite heavy and if you haven't used them up before coming home, you can always throw them away - thaty will free some weight for the goodies you want to bring back!
Saying all that, I think it depends who you fly with. We came back from Menorca in June and my friend was 6kgs over the weight allowance and nobody batted an eyelid or charged her a penny.