How to beat the 'Israel did the same thing' argument

I'll bet a lot of you have been arguing with friends, acquaintances, work colleagues and obnoxious twits over the 'Palestinian' intention to declare a 'state' unilaterally via the UN General Assembly in September. And I'm sure that a lot of you have come across the argument that "Israel declared a state in 1947 based on a General Assembly vote, so why can't the 'Palestinians' do the same thing